U.S. patent application number 14/705152 was filed with the patent office on 2017-01-12 for fuel modifiers for natural gas reciprocating engines.
The applicant listed for this patent is CHEVRON U.S.A. INC.. Invention is credited to Amir Gamal Maria, Malika Mukundan, John Bradley Roucis, William Lawrence Schinski, John Gerard Wolff.
Application Number | 20170009166 14/705152 |
Document ID | / |
Family ID | 53298584 |
Filed Date | 2017-01-12 |
United States Patent
Application |
20170009166 |
Kind Code |
A1 |
Roucis; John Bradley ; et
al. |
January 12, 2017 |
FUEL MODIFIERS FOR NATURAL GAS RECIPROCATING ENGINES
Abstract
Described herein are fuel modifiers for natural gas
reciprocating engines, while recognizing the application of the
inventions herein may be applied more broadly, to other natural
gas-based engine systems. The fuel modifiers are primarily
free-radical initiators, and the presence of this fuel modifier
allows the engine operator to operate the engine under leaner
conditions because, while employing the same ignition energy, more
free-radicals are formed, thus overcoming the problems associated
with dilution of the pool of free-radicals in the flame.
Inventors: |
Roucis; John Bradley;
(Hercules, CA) ; Maria; Amir Gamal; (Vallejo,
CA) ; Schinski; William Lawrence; (San Rafael,
CA) ; Mukundan; Malika; (Houston, CA) ; Wolff;
John Gerard; (Houston, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CHEVRON U.S.A. INC. |
San Ramon |
CA |
US |
|
|
Family ID: |
53298584 |
Appl. No.: |
14/705152 |
Filed: |
May 6, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61989936 |
May 7, 2014 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C10L 3/10 20130101; C10L
3/003 20130101; C10L 2200/0263 20130101; C10L 3/06 20130101; C10L
10/02 20130101; C10L 2200/0259 20130101; C10L 2230/22 20130101 |
International
Class: |
C10L 3/00 20060101
C10L003/00; C10L 10/02 20060101 C10L010/02; C10L 3/06 20060101
C10L003/06 |
Claims
1. A modified fuel, comprising a base fuel and a fuel modifier
selected from the group consisting of azo compounds,
dialkylsulfides, alkyl sulfones, nitroalkyls, peroxygens, and
hydrocarbons which contain symmetrically substituted carbon-carbon
bonds where that bond is relatively weak.
2. The modified fuel of claim 1, wherein the base fuel is natural
gas.
3. The modified fuel of claim 2, wherein the fuel modifier is
selected from the group consisting of azo compounds having a
molecular weight of between 80 and 500 g/mol, and a decomposition
temperature between 80 and 300.degree. C.
4. The modified fuel of claim 2, wherein the fuel modifier is
selected from the group consisting of azobisisoalkyls represented
by structures (1) and (2): ##STR00006## wherein R.sub.1 through
R.sub.4 are each independently selected from the group consisting
of hydrogen; hydroxyl; methyl; 2-cyanoprop-2-yl; and linear or
branched, substituted or unsubstituted C.sub.1-C.sub.15 alkyl
groups, C.sub.1-C.sub.15 alkenyl groups, C.sub.1-C.sub.15
hydroxyalkyl groups, C.sub.1-C.sub.15 alkoxyalkyl groups,
C.sub.1-C.sub.15 aminoalkyl groups, C.sub.1-C.sub.15 carboxyalkyl
groups, C.sub.1-C.sub.15 aminocarboxyalkyl groups and
C.sub.1-C.sub.15 hydroxycarboxyalkyl groups.
5. The modified fuel of claim 4, wherein the fuel modifier is
azobisisobutylnitrile.
6. The modified fuel of claim 2, wherein the fuel modifier is
selected from the group consisting of dialkylsulfides represented
by structure (3): ##STR00007## wherein R.sub.5 and R.sub.6 are each
independently selected from the group consisting of hydrogen;
hydroxyl; methyl; and linear or branched, substituted or
unsubstituted C.sub.1-C.sub.15 alkyl groups, C.sub.1-C.sub.15
alkenyl groups, C.sub.1-C.sub.15 hydroxyalkyl groups,
C.sub.1-C.sub.15 alkoxyalkyl groups, C.sub.1-C.sub.15 aminoalkyl
groups, C.sub.1-C.sub.15 carboxyalkyl groups, C.sub.1-C.sub.15
aminocarboxyalkyl groups, C.sub.1-C.sub.15 hydroxycarboxyalkyl
groups, C.sub.1-C.sub.15 aryl groups, and C.sub.1-C.sub.15
alkylaryl groups.
7. The modified fuel of claim 6, wherein the fuel modifier is
dismethylsulfide.
8. The modified fuel of claim 2, wherein the fuel modifier is
selected from the group consisting of selected from the group
consisting of sulfones represented by structure (4): ##STR00008##
wherein R.sub.7 and R.sub.8 are each independently selected from
the group consisting of hydrogen; hydroxyl; methyl; and linear or
branched, substituted or unsubstituted C.sub.1-C.sub.15 alkyl
groups, C.sub.1-C.sub.15 alkenyl groups, C.sub.1-C.sub.15
hydroxyalkyl groups, C.sub.1-C.sub.15 alkoxyalkyl groups,
C.sub.1-C.sub.15 aminoalkyl groups, C.sub.1-C.sub.15 carboxyalkyl
groups, C.sub.1-C.sub.15 aminocarboxyalkyl groups and
C.sub.1-C.sub.15 hydroxycarboxyalkyl groups.
9. The modified fuel of claim 8, wherein the fuel modifier is
dismethylsulfone.
10. The modified fuel of claim 2, wherein the fuel modifier is
selected from the group consisting of nitro compounds represented
by structure (5): ##STR00009## wherein R.sub.9 is selected from the
group consisting of hydrogen; hydroxyl; methyl; and linear or
branched, substituted or unsubstituted C.sub.1-C.sub.15 alkyl
groups, C.sub.1-C.sub.15 alkenyl groups, C.sub.1-C.sub.15
hydroxyalkyl groups, C.sub.1-C.sub.15 alkoxyalkyl groups,
C.sub.1-C.sub.15 aminoalkyl groups, C.sub.1-C.sub.15 carboxyalkyl
groups, C.sub.1-C.sub.15 aminocarboxyalkyl groups, C.sub.1-C.sub.15
hydroxycarboxyalkyl groups, C.sub.1-C.sub.15 aryl groups, and
C.sub.1-C.sub.15 alkylaryl groups.
11. The modified fuel of claim 10, wherein the fuel modifier is
nitromethane.
12. The modified fuel of claim 2, wherein the fuel modifier is
selected from the group consisting of peroxygens compounds
represented by structure (6): ##STR00010## wherein R.sub.10 and
R.sub.11 are independently selected from the group consisting of
hydrogen; methyl; and linear or branched, substituted or
unsubstituted C.sub.1-C.sub.15 alkyl groups, C.sub.1-C.sub.15
alkenyl groups, C.sub.1-C.sub.15 hydroxyalkyl groups,
C.sub.1-C.sub.15 alkoxyalkyl groups, C.sub.1-C.sub.30 carboxyalkyl
groups, C.sub.1-C.sub.30 hydroxycarboxyalkyl groups,
C.sub.1-C.sub.15 aryl, and C.sub.1-C.sub.15 alkylaryl groups.
13. The modified fuel of claim 12, wherein the fuel modifier is
selected from the group consisting of linear and branched
C.sub.1-C.sub.5 alkyl peroxides.
14. The modified fuel of claim 13, wherein the fuel modifier is
di-tert-butyl peroxide.
15. The modified fuel of claim 13, wherein the fuel modifier is
dimethoxymethane.
16. The modified fuel of claim 2, wherein the fuel modifier is
selected from the group represented by the formula below:
R.sub.13--C(R.sub.14, R.sub.12)--C(R.sub.14, R.sub.12, R.sub.13)
wherein R.sub.12, R.sub.13, and R.sub.14 are independently selected
from the group consisting of hydrogen; methyl; and linear or
branched, substituted or unsubstituted C.sub.1-C.sub.15 alkyl
groups, C.sub.1-C.sub.15 alkenyl groups, C.sub.1-C.sub.15
hydroxyalkyl groups, C.sub.1-C.sub.15 alkoxyalkyl groups,
C.sub.1-C.sub.30 carboxyalkyl groups, C.sub.1-C.sub.30
hydroxycarboxyalkyl groups, C.sub.1-C.sub.15 aryl, and
C.sub.1-C.sub.15 alkylaryl groups. Exemplary hydrocarbon compounds
which contain symmetrically substituted carbon-carbon bonds include
2,3-dimethyl-2,3-diphenylbutane.
Description
FIELD OF THE INVENTION
[0001] Described herein are fuel modifiers for natural gas powered
reciprocating engines. The fuel modifiers described herein are
free-radical initiators and are particularly suited for optical
ignition (i.e. laser-spark ignition) natural gas engines. The
application of suitable combustion enhancers can allow for the use
of lower-powered lasers. Other advantages of fuel modifiers for
enhanced combustion include the ability for lean engine operation
for reduced emissions, and improved fuel efficiencies.
[0002] Fuel modifiers can also reduce the engine's knock
tendencies, which can increase the power density and/or improve
engine efficiency. The fuel modifiers described below are also
applicable to traditional spark ignition natural gas engines, and
to other modes of natural gas engine operations, e.g. hybrids and
turbines, and others, as well as to alternate ignition systems for
reciprocating natural gas engines, such as microwave energy pulse
systems, sonic systems, and others.
BACKGROUND OF THE INVENTION
[0003] Natural gas fueled reciprocating engines are engines that
use natural gas as a fuel source. Large natural gas reciprocating
engines are typically used in stationary applications, such as for
use to generate electricity. Commercially available stationary
natural gas engines typically have up to 20 megawatt capacities,
and 10-20 cylinders per engine.
[0004] The most common ignition source for natural gas engines is
spark ignition. In general, oxygen-containing air and the natural
gas are mixed upstream from the engine cylinder. The fuel/air
mixture is then fed into the engine cylinder, during the intake
stroke, and ignited by a spark plug (power stroke).
[0005] There is increasing interest in operating natural gas
engines under lean conditions, meaning the air to fuel ratio is in
excess, with respect to the air content, of the stoichiometric
air-to-fuel ratio. As a fuel mixture becomes leaner, the combustion
temperature is reduced, therefore reducing NO.sub.x emissions as
such emissions form at higher combustion temperatures. Leaner
operation also improves the engine efficiency by reducing heat
losses and creating a more thermodynamically favorable mixture
composition. However, as a fuel mixture becomes leaner, the spark
energy needed for combustion increases as well, and it becomes
difficult to initiate a flame front.
[0006] NO.sub.x emissions from stationary natural gas engines are
regulated emissions, meaning producers are either limited in the
amount of NO.sub.x emissions that their engines can emit, or must
pay regulatory fines if their emissions sources exceed mandated
thresholds.
[0007] Regulating authorities have, or are in the process of,
mandating NO.sub.x emissions limits which are below those that can
be achieved practically with current natural gas engines. Further,
existing spark natural gas engines are becoming increasingly
expensive to maintain, as leaner operating conditions are applied.
This cost escalation is caused by the producer's need to run
increasingly leaner mixtures, which reduces the cycle life of the
spark plugs. As a result, most producers are now changing the spark
plugs on their stationary engines about every 100 hours of
operation. Therefore, there is a current need for an ultra-lean
natural gas fuel with an ignition threshold that can be reached
using reduced spark energy, thereby eliminating the need to run
high-voltage spark plugs at their upper limits.
[0008] The power output of natural gas engines is limited by the
amount of fuel that can be admitted before pressure waves form in
the cylinder (referred to as engine knock). Therefore, there is a
current need for a natural gas fuel that reduces the engine's knock
tendencies. This would allow for a higher power density. In lieu of
a higher power density, a higher compression ratio would be
possible with a natural gas fuel that reduces the engine's knock
tendencies. This would then lead to higher engine efficiencies. A
higher power density and/or a higher compression ratio will
increase the engine's peak pressure.
[0009] A higher peak pressure for existing spark ignition engines
further reduces the cycle life of the spark plugs. This factor
increases the operating costs, for spark ignition engines.
[0010] To accommodate the need to run natural gas engines under
increasingly leaner conditions, and to run at higher peak
pressures, the stationary natural gas engine industry will need to
migrate from spark ignition to more advanced ignition systems.
Laser ignition provides greater spark energy as compared to high
voltage spark plugs currently in commercial use. Unlike spark
ignition, laser ignition also operates more reliably at higher
cylinder pressures. However, laser ignition systems suffer from
issues not exhibited by traditional spark systems.
[0011] First, the laser ignition pulse beam is introduced into the
combustion chamber through a window. During operation, particulate
combustion products settle on the window, impeding or scattering
the beam being transmitted into the combustion chamber.
[0012] Second, the high-power laser systems currently contemplated
are not commercially viable as they are costly and the beams they
produce are too powerful for fiber optic cable transmission. Fiber
optic cables provide the opportunity to allow an engine to have a
single laser beam source, the beam from which is then split and
distributed to each cylinder via fiber optic cables. As the natural
gas engine industry moves to lower cost, less powerful laser
sources, there is a need for a natural gas fuel that can be ignited
using the beams from these lower power lasers.
[0013] Accordingly, there is a current need for a natural gas fuel
containing fuel modifiers which give an ignition threshold allowing
ultra-lean operation which is achieved using either reduced spark
energy (if a spark plug is used), or reduced laser energy (if laser
ignition is used). There is also a current need for a modified
natural gas fuel which reduces the engine's knock tendencies,
thereby allowing a higher power density and/or increased
compression ratio, which would improve engine efficiency.
SUMMARY OF THE INVENTION
[0014] Described herein are fuel modifiers for natural gas
reciprocating engines, while recognizing the application of the
inventions herein may be applied more broadly, to other natural
gas-based engine systems.
[0015] The fuel modifiers are primarily free-radical initiators,
and the presence of this fuel modifier allows the engine operator
to operate the engine under leaner conditions because, while
employing the same ignition energy, more free-radicals are formed,
thus overcoming the problems associated with dilution of the pool
of free-radicals in the flame. The same principle can allow the
engine operator to operate at a lower ignition energy. Altering the
source of free-radical initiators can also reduce the engine's
knock tendency, which can increase engine performance and
efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a plot of the combustion efficiency (x) as a
function of the equivalence ratio (.phi.) for the baseline fuel,
methane/di-tert-butyl peroxide (DTBP), methane/nitromethane (NM),
and methane/methylal modified fuels.
[0017] FIG. 2 is a plot of the combustion probability as a function
of the minimum ignition energy (MIE) for the baseline fuel and a
methane/DTBP fuels.
[0018] FIG. 3 is a plot of the combustion probability as a function
of the minimum ignition energy (MIE) for the baseline fuel and a
methane/NM fuels.
[0019] FIG. 4 is a plot of the combustion probability as a function
of the minimum ignition energy (MIE) for the baseline fuel and a
methylal/NM fuels.
DETAILED DESCRIPTION OF THE INVENTION
Introduction
[0020] "Periodic Table" refers to the version of IUPAC Periodic
Table of the Elements dated Jun. 22, 2007, and the numbering scheme
for the Periodic Table Groups is as described in Chemical and
Engineering News, 63(5), 27 (1985).
[0021] "Natural gas" means a predominantly methane-based fuel that
may contain other hydrocarbons such as ethane and propane, and
other compounds.
[0022] Where permitted, all publications, patents and patent
applications cited in this application are herein incorporated by
reference in their entirety; to the extent such disclosure is not
inconsistent with the modified fuels described herein.
[0023] Unless otherwise specified, the recitation of a genus of
elements, materials or other components, from which an individual
component or mixture of components can be selected, is intended to
include all possible sub-generic combinations of the listed
components and mixtures thereof. Also, "include" and its variants
are intended to be non-limiting, such that recitation of items in a
list is not to the exclusion of other like items that may also be
useful in the materials, compositions and methods of this
invention.
Lowering Ignition Energy Requirements and Knock Tendencies via
Free-Radical Initiators
[0024] Reciprocating internal combustion engines take a mixture of
a hydrocarbon fuel (e.g. methane) and air, compress the fuel/air
mixture, and then use optical or spark ignition to ignite the
compressed fuel/air mixture. The term "reciprocating" refers to the
motion of the engine's crank mechanism. The reciprocating engine
employs a crank-slider mechanism, where the slider is the piston.
Note that other natural gas engine configurations, e.g. gas
turbines, and others, can benefit from the introduction of suitable
combustion enhancers as well.
[0025] The piston is moved up and down within a combustion chamber
by the rotary motion of a piston arm, to which, mechanical energy
is transferred from the piston arm. Rotation of the crankshaft
makes the piston move up and down within the combustion chamber,
and thereby the crankshaft extracts mechanical energy. Intake
valves on the top of the combustion chamber allow for the
introduction of a hydrocarbon fuel/air mixture, and exhaust valves
allow for the release of residual combustion products.
[0026] Combustion of the fuel/air mixture (the process of
converting the fuel/air mixture to carbon dioxide and water)
involves a three step chain reaction: (1) initiation, (2)
propagation, and (3) termination. During initiation, the reactant
components decompose into free-radical species. During propagation,
free-radical/molecule reactions form additional free-radical
species, and this process continues until all of the free-radicals
have reacted to form carbon dioxide, water and residual components
such as carbon and unreacted fuel (termination).
[0027] Initiation begins with the decomposition of a small portion
of the fuel by the optical or spark ignition source, or other
suitable discrete ignition source. The leaner the engine is
operated (i.e. at lower fuel: air ratios), the pool of free-radical
species in the flame becomes more dilute because of the fixed
amount of ignition energy available to convert the fuel into
free-radical species. This, in turn, could reduce the rate of flame
propagation resulting in less efficient engine operation.
[0028] Described herein, as follows, are fuel modifiers for natural
gas powered reciprocating engines. The fuel modifiers described
herein are particularly suited for optical ignition (i.e. laser
ignition) natural gas engines, while also being applicable to
traditional spark ignition systems. The application of suitable
combustion enhancers can allow for the use of lower-powered lasers.
Other advantages of fuel modifiers for enhanced combustion include
the ability for lean engine operation for reduced emissions, and
improved fuel efficiencies. Fuel modifiers can also reduce the
engine's knock tendencies, which can increase the power density
and/or improve engine efficiency. The fuel modifiers described
below are also applicable to traditional spark ignition natural gas
engines, and to other modes of natural gas engine operations, e.g.
hybrids and turbines, and others, as well as to alternate ignition
systems for reciprocating natural gas engines, such as microwave
energy pulse systems, sonic systems, and others.
[0029] The fuel modifiers described herein are free-radical
initiators and are characterized as having a decomposition
activation energy that is generally lower than the fuel. In one
instance, the presence of the fuel modifier allows the engine
operator to operate the engine under leaner conditions because,
while employing the same ignition energy, more free-radicals are
formed, thus overcoming the problems associated with dilution of
the pool of free-radicals in the flame.
[0030] In a second instance, the presence of the fuel modifier
allows the engine operator to operate using lower ignition energy.
This is because the fuel modifier allows for the same concentration
of free-radical species in the flame, using less ignition energy.
Lowering the ignition energy lengthens the life of spark plugs used
in the engines. For optical ignition reciprocating engines, a lower
energy laser light source can be employed, allowing, for example,
the use of a single beam source that can be split and routed to
each cylinder via optical fiber. Currently, the ignition energy
needed to operate engines employing hydrocarbon fuels such as
methane dictate the use a relatively high-energy beam source for
each cylinder.
[0031] The advantages of laser ignition can be summarized as
follows. Laser ignition can offer ignition of mixtures at higher
pressures, and higher compression ratios, resulting in higher
engine efficiencies. Also, laser ignition can give improved
ignition of leaner mixtures, giving lower NOx emissions, as well as
ignition of lower quality fuel-air mixtures (e.g. synthesis gas,
sewer gas, and landfill gas). Laser ignition systems may offer
lower maintenance requirements compared to spark plugs, in that
maintenance of the spark gap is not required. Finally,
"multi-point" ignition within the combustion volume is possible
with lasers, yielding higher burn rates.
[0032] The selection of free-radical initiators as a fuel modifier
can be influenced by choice of the laser ignition source frequency,
wherein the pairing of the laser (fundamental) ignition frequency
to the free-radical initiator compound can be optimally chosen.
This "pairing" can be optimized in a way that the resonant
frequency of the initiator is close to the fundamental frequency of
the laser, thereby enhancing free-radical breakdown affecting
ignition of the fuel/air mixture in an optimal way.
[0033] Fuel modifiers useful herein as free-radical initiators are
selected from the group consisting of azo compounds,
dialkylsulfides, alkyl sulfones, nitroalkyls, peroxygens (e.g.
peracids, hydroperoxides, dialkyl/alkyl-aryl peroxides, diaryl
peroxides, and mixtures thereof), and hydrocarbons which contain
symmetrically substituted carbon-carbon bonds where that bond is
relatively weak.
[0034] Fuels modified with the fuel modifiers described herein will
contain a sufficient mole percent of fuel modifier to reduce the
minimum ignition energy (MIE), which is recorded at 80% probability
of ignition (MIE.sub.80), by between 40% and 70%. The reduction was
observed in a rapid compression machine (RCM) and could be larger
in an engine.
[0035] In one embodiment, a modified fuel is provided containing a
sufficient mole percent fuel modifier to reduce the fuel-to-air
ratio by over 14%. In another embodiment is provided a modified
fuel having between 0.01 and 10 mole percent fuel modifier. In one
subembodiment, the fuel contains between 0.01 and 5 percent fuel
modifier. In another subembodiment, the fuel contains between 0.01
and 1 percent fuel modifier.
[0036] In one embodiment, the free-radical initiator is selected
from azo compounds having a molecular weight of between 80 and 500
g/mol, and a decomposition temperature between 80 and 300.degree.
C.
[0037] The azo compounds which may be employed as fuel modifiers
are selected from the group consisting of azobisisoalkyls
represented by structures (1) and (2):
##STR00001##
wherein structure (2) is referred to as, "azoxy," and, wherein
R.sub.1 through R.sub.4 are each independently selected from the
group consisting of hydrogen; hydroxyl; methyl; 2-cyanoprop-2-yl,
and linear or branched, substituted or unsubstituted
C.sub.1-C.sub.15 alkyl groups, C.sub.1-C.sub.15 alkenyl groups,
C.sub.1-C.sub.15 hydroxyalkyl groups, C.sub.1-C.sub.15 alkoxyalkyl
groups, C.sub.1-C.sub.15 aminoalkyl groups, C.sub.1-C.sub.15
carboxyalkyl groups, C.sub.1-C.sub.15 aminocarboxyalkyl groups and
C.sub.1-C.sub.15 hydroxycarboxyalkyl groups.
[0038] Exemplary azo compounds include azobisisobutylnitrile.
[0039] The dialkylsulfides compounds which may be employed as fuel
modifiers are selected from the group consisting of dialkylsulfides
represented by structure (3):
##STR00002##
wherein R.sub.5 and R.sub.6 are each independently selected from
the group consisting of hydrogen; hydroxyl; methyl; and linear or
branched, substituted or unsubstituted C.sub.1-C.sub.15 alkyl
groups, C.sub.1-C.sub.15 alkenyl groups, C.sub.1-C.sub.15
hydroxyalkyl groups, C.sub.1-C.sub.15 alkoxyalkyl groups,
C.sub.1-C.sub.15 aminoalkyl groups, C.sub.1-C.sub.15 carboxyalkyl
groups, C.sub.1-C.sub.15 aminocarboxyalkyl groups, C.sub.1-C.sub.15
hydroxycarboxyalkyl groups, C.sub.1-C.sub.15 aryl groups, and
C.sub.1-C.sub.15 alkylaryl groups.
[0040] Exemplary dialkylsulfides compounds include
dimethylsulfide.
[0041] The alkyl sulfones compounds which may be employed as fuel
modifiers are selected from the group consisting of sulfones
represented by structure (4):
##STR00003##
wherein R.sub.7 and R.sub.8 are each independently selected from
the group consisting of hydrogen; hydroxyl; methyl; and linear or
branched, substituted or unsubstituted C.sub.1-C.sub.15 alkyl
groups, C.sub.1-C.sub.15 alkenyl groups, C.sub.1-C.sub.15
hydroxyalkyl groups, C.sub.1-C.sub.15 alkoxyalkyl groups,
C.sub.1-C.sub.15 aminoalkyl groups, C.sub.1-C.sub.15 carboxyalkyl
groups, C.sub.1-C.sub.15 aminocarboxyalkyl groups and
C.sub.1-C.sub.15 hydroxycarboxyalkyl groups.
[0042] Exemplary sulfones compounds include dimethylsulfone.
[0043] The nitroalkyl compounds which may be employed as fuel
modifiers are selected from the group consisting of nitro compounds
represented by structure (5):
##STR00004##
wherein R.sub.9 is selected from the group consisting of hydrogen;
hydroxyl; methyl; and linear or branched, substituted or
unsubstituted C.sub.1-C.sub.15 alkyl groups, C.sub.1-C.sub.15
alkenyl groups, C.sub.1-C.sub.15 hydroxyalkyl groups,
C.sub.1-C.sub.15 alkoxyalkyl groups, C.sub.1-C.sub.15 aminoalkyl
groups, C.sub.1-C.sub.15 carboxyalkyl groups, C.sub.1-C.sub.15
aminocarboxyalkyl groups, C.sub.1-C.sub.15 hydroxycarboxyalkyl
groups, C.sub.1-C.sub.15 aryl groups, and C.sub.1-C.sub.15
alkylaryl groups.
[0044] Exemplary nitroalkyl compounds include nitromethane.
[0045] The peroxygen compounds which may be employed as fuel
modifiers are selected from the group consisting of peroxygens
compounds represented by structure (6):
##STR00005##
wherein R.sub.10 and R.sub.11 are independently selected from the
group consisting of hydrogen; methyl; and linear or branched,
substituted or unsubstituted C.sub.1-C.sub.15 alkyl groups,
C.sub.1-C.sub.15 alkenyl groups, C.sub.1-C.sub.15 hydroxyalkyl
groups, C.sub.1-C.sub.15 alkoxyalkyl groups, C.sub.1-C.sub.30
carboxyalkyl groups, C.sub.1-C.sub.30 hydroxycarboxyalkyl groups,
C.sub.1-C.sub.15 aryl, and C.sub.1-C.sub.15 alkylaryl groups.
[0046] Exemplary peroxygen compounds include linear and branched
C.sub.1-C.sub.5 alkyl peroxides such as di-tent-butyl peroxide and
dimethoxymethane.
[0047] Hydrocarbons which contain symmetrically substituted
carbon-carbon bonds where that bond is relatively weak, may be used
as fuel modifiers. These type of compounds which may be employed as
initiators, or combustion enhancers, are selected from the group
represented by the formula below:
R.sub.13--C(R.sub.14, R.sub.12)--C(R.sub.14, R.sub.12,
R.sub.13)
wherein R.sub.12, R.sub.13, and R.sub.14 are independently selected
from the group consisting of hydrogen; methyl; and linear or
branched, substituted or unsubstituted C.sub.1-C.sub.15 alkyl
groups, C.sub.1-C.sub.15 alkenyl groups, C.sub.1-C.sub.15
hydroxyalkyl groups, C.sub.1-C.sub.15 alkoxyalkyl groups,
C.sub.1-C.sub.30 carboxyalkyl groups, C.sub.1-C.sub.30
hydroxycarboxyalkyl groups, C.sub.1-C.sub.15 aryl, and
C.sub.1-C.sub.15 alkylaryl groups. Exemplary hydrocarbon compounds
which contain symmetrically substituted carbon-carbon bonds include
2,3-dimethyl-2,3-diphenylbutane.
Modified Fuels and Engine Delivery Systems
[0048] Methane-based fuels containing one or more of the fuel
modifiers described herein can be manufactured by blending the base
fuel and fuel modifiers at a location remote from the engine, or
proximal to the engine. An example of remote blending would be
where the fuel and modifiers are combined at a blending station
located at a refinery or local terminal. Methods for blending
methane-based fuels (e.g. natural gas) and hydrocarbon components
are known.
[0049] Depending on the modifiers employed (e.g. where a particular
modifier is prone to settling during delivery), it may be necessary
to blend modifiers at different locations.
[0050] Where the blending occurs proximal to the engine, the fuel
and modifiers can be blended, for example, at the location where
the engines are located. In an instance where multiple engines are
located at a facility, which is common for most electrical power
generation stations, the blending can occur at a single point at
the facility, and the modified fuel can then be conveyed, e.g. via
piping, to each individual engine.
[0051] Where one or more engines are located at a single facility,
each engine has a blending system for blending the methane-based
fuel with one or more modifiers. Depending on the nature of the
modifiers, multiple blending systems may be employed for blending
the modifiers at optimal points in the fuel delivery system.
[0052] In one embodiment, one or more fuel modifiers are blended
with the methane-based fuel proximal to each engine combustion
chamber. In another embodiment, one or more fuel modifiers are
injected directly into each engine combustion chamber. The direct
injection can be directed in a manner that optimizes the used of
the fuel modifier, including targeting the ignition source.
[0053] Embodiments where the fuel modifiers are blended for each
engine, or blended for each combustion chamber, are advantageous as
these systems allow the user to tailor the fuel/modifier blend to
accommodate the mode of operation of the engine (e.g. lean
combustion, ignition source and energy, required emission outputs,
and the like).
[0054] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made without departing from the spirit and scope of the
invention.
EXAMPLES
[0055] Methane/air mixtures and methane/fuel modifier mixtures were
ignited via laser ignition at elevated pressures and temperatures
for conditions representative of internal combustion engines. The
experiments measured the lean limit and minimum ignition energy at
the different test conditions.
[0056] A rapid compression machine (RCM) was used to simulate the
rapid pressure and temperature rises typical of internal combustion
engines and a Nd:YAG laser was employed as the ignition source.
RCMs are traditionally used for fuel characterization and chemical
kinetics studies of hydrocarbons spanning from simple fuels like
methane to larger species in the diesel range.
[0057] The construction and operation of the RCM is further
described in two publications. (See, "Laser Ignition of Methane-Air
Mixtures with a Rapid Compression Machine" 53rd AIAA Aerospace
Sciences Meeting, Jan. 5, 2015, eISBN: 978-1-62410-343-8). (See
also, "Fundamental Studies of Laser Ignition of Natural Gas/Air
Mixtures at Elevated Temperatures and Pressures" 9th U.S. National
Combustion Meeting May 17-20, 2015)
[0058] The RCM employed is an opposed piston system mechanically
similar to the well characterized machine at the National
University of Ireland, Galway. The RCM can operate with compression
ratios from .about.10-14:1, maximum compressed pressures of
.about.50 bars, and compression times of 15-25 ms. For the
experiments presented herein, the nominal operating conditions were
initial pressures of 1 bar, compression ratio of 11.6:1 and
compression times of 20 ms which led to compressed pressures of 30
bars.
[0059] A Nd:YAG (BigSky ULTRA) at 1064 nm with a beam quality
M.sub.2=1.9 and a laser pulse duration of 12 ns was used as the
laser source. The laser beam was first passed through a variable
attenuator consisting of a waveplate and a polarizer. The variable
attenuator enables the control of laser energy. A pair of beam
splitters are used to allow a small fraction of the laser beam to
reach the energy meter and a photodiode. This allows for pulse
energy monitoring and provides accurate information about the
timing of the laser firing. The remainder of the laser beam is
transmitted by the first beam splitter and is steered using a pair
of mirrors into the RCM combustion chamber. The beam is then
focused through an optical plug using a plano-convex lens (f=25 mm)
located inside the plug. Optical breakdown is achieved at the focus
of the lens and the resulting plasma is used to ignite the fuel
mixture. A second photodiode and a band-pass filter are employed at
the other end of the chamber to detect the formation of a spark
created by the laser pulse. The band-pass filter blocks the 1064 nm
laser light such that only the plasma radiation is transmitted. The
laser and the RCM timing are achieved by using a pulse delay
generator (BNC 555).
[0060] The first step in RCM operation is to establish a vacuum in
the pneumatic drive chambers to draw the pistons into their
retracted positions. Next, the combustion chamber is twice flushed
with nitrogen and placed under vacuum to remove any contaminants or
combustion products from previous combustion events. Once the
combustion chamber has reached sufficient vacuum (<3 mbar) the
fuel/oxidizer mixture is introduced. The blend is then allowed to
mix for approximately five minutes prior to compression. Next, the
locking chambers are pressurized with hydraulic fluid to lock the
pistons in place. Once the pistons are locked, the pneumatic drive
chambers are charged with high pressure air and the RCM is now
ready to fire. Upon firing, the hydraulic pressure in the locking
chambers is rapidly released allowing the pneumatic pressure in the
drive cylinders to propel the pistons and rapidly compress the
fuel/oxidizer mixture. As the pistons approach the end of their
stroke, the forcing of hydraulic fluid through passageways slows
them, and the pneumatic pressure of the drive cylinders holds the
pistons in place until the end of the experiment.
[0061] In typical RCM auto-ignition experiments, initial conditions
are selected such that the fuel-oxidizer mixture auto-ignites
within 5 to 200 ms after the end of compression, thus allowing
ignition delays to be measured from pressure traces. During the
ignition delay period, the temperature of the mixture decreases
because the RCM is not completely adiabatic. Accordingly, if the
ignition delay period is sufficiently long, the mixture will never
ignite. For the experiments in this study, mixtures were chosen
with sufficiently long ignition delay periods such that they would
not auto-ignite in the RCM. Rather, an external ignition source
(laser spark) was required to initiate the combustion.
[0062] Laser ignition of methane/air inside the RCM was
investigated at various equivalence ratios (fuel-to-air ratio,
.phi.). In order to understand the lean limit the fuel energy was
kept constant while the equivalence ratio was varied. This
corresponds to a more realistic scenario in which a real engine
will operate (which is also more consistent with real engine
operation where varying boost is used to keep the fuel energy
constant). Laser energy and laser firing time were fixed at
constant values of 5 mJ and 26 ms for all lean limit experiments.
It should be noted that none of the test conditions used resulted
in auto-ignition for any of the tests described herein.
TABLE-US-00001 TABLE 1 Fuel-to-Air Initial Initial Pressure After
Equivalence Pressure Temperature Compression Ratio - .phi. (bar)
(K) (bar) 0.6 1 320 28.4 0.55 1.086 319 30.9 0.5 1.188 318 34 0.44
1.342 317 38.5 0.42 1.4 316 40.2 0.25 2.314 312 67.4 0.6 1 320
28.4
[0063] The results of the lean limit study presented in FIG. 1 are
expressed in terms of the combustion efficiency, x, which we define
as the ratio of the net heat released (Q.sub.net) to the energy
content of the fuel charge (E.sub.methane and modifier). Stated
differently, the combustion efficiency represents the percent of
the fuel that successfully combusts at a particular fuel-to-air
ratio (.phi.).
[0064] The heat release is computed based on the RCM's pressure
data analysis as described by Heywood, "Internal Combustion Engines
Fundamentals", New York, McGraw Hill; 1988. If complete combustion
is achieved, the net heat release should match the fuel energy.
However, as we sweep through leaner equivalence ratios, the amount
of heat released during the combustion process decreases (i.e.
.chi. goes down with decreasing equivalence ratio). Acknowledging
that operation conditions might vary in a real engine we are
defining the lean limit in a statistical sense by reporting the
lean limit corresponding to .phi..sub.90=0.45, .phi..sub.50=0.39,
.phi..sub.20=0.36 where, for example, .phi..sub.90 denotes the
minimum .phi. value where x=0.9(=90%).
[0065] A baseline fuel mixture consisting of methane was first
investigated. Modified fuels were investigated by adding 1% molar
percent of each of the following fuel modifiers to methane:
di-tert-butyl peroxide (DTBP); nitromethane (NM) and methylal
(dimethoxymethane/DMM).
[0066] FIG. 1 is a plot of the combustion efficiency (x) as a
function of the equivalence ratio (.phi.) for the baseline fuel,
methane/DTBP, methane/NM, and methane/methylal modified fuels.
[0067] As indicated in FIG. 1, to achieve 80% combustion
efficiency, the methane/air fuel required a fuel-to-air ratio of
about 0.42, whereas the fuels containing DTBP, NM, and DMM achieved
80% efficiency at the more favored leaner ratios of 0.36. In other
words, the lean limit extended by over 14% at a fixed combustion
efficiency of 80%.
[0068] Another important parameter that characterizes laser and
spark ignition is the minimum ignition energy (MIE). A fundamental
understanding of the MIE is of extreme practical importance because
a decrease in minimum ignition energy could enable the use of
optical fibers for delivering laser radiation to the engines.
[0069] For MIE we plot the probability of successful ignition
(based on multiple tests) versus laser energy. The test conditions
for the MIE investigation are indicated in Table 2 below.
TABLE-US-00002 TABLE 2 Fuel-to-Air Initial Temperature After Laser
Firing Equivalence Pressure Compression Temperature Ratio - .phi.
(bar) (K) (K) 0.4 1 750 560
[0070] As indicated in FIG. 2, to achieve a MIE.sub.90 (90%
probability of ignition) for the methane/air fuel baseline mixture,
a 7.2 mJ laser spark was needed. In contrast, as indicated in Table
3 below, the laser spark energy needed to achieve a MIE.sub.90 for
the methane/DTBP modified fuels was lower.
TABLE-US-00003 TABLE 3 Fuel MIE.sub.90 (mJ) methane/air 7.2
methane/0.2 DTBP 4.6 methane/1% DTBP 4.5
[0071] Referring to FIG. 3, methane/NM modified fuels were tested
containing 0.2%, 1% and 5% NM. The results are indicated in Table 4
below. As indicated in the table below, fuels modified with NM had
MIE.sub.90 laser spark energy values that were lower than the
methane/air baseline fuel mixture.
TABLE-US-00004 TABLE 4 Fuel MIE.sub.90 (mj) methane/air 7.2
methane/0.2% NM 4.6 methane/1% NM 2.6 methane/5% NM 2.1
[0072] Referring to FIG. 4, methane/DMM modified fuels were tested
containing 0.04%, 0.2% and 1% DMM. The results are indicated in
Table 5 below. As indicated in the table below, except at the very
low concentration of 0.04%, fuels modified with DMM had MIE.sub.90
laser spark energy values that were lower than the methane/air
baseline fuel mixture.
TABLE-US-00005 TABLE 5 Fuel MIE.sub.90 (mj) methane/air 7.2
methane/0.04% DMM 7.2 methane/0.2% DMM 4.0 methane/1% DMM 4.0
* * * * *