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Effect of Hydrogen Enriched Hydrocarbon Combustion on Emissions and Performance
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ABSTRACT
The principle of this mode of combustion is to add a
percentage of hydrogen gas to the combustion reactions
of either compression or spark ignition engines. The
addition of hydrogen has been shown to decrease the
formation of NOx, CO and unburned hydrocarbons.
Studies have shown that added hydrogen in
percentages as low as 5-10% percent of the
hydrocarbon fuel can reduce that hydrocarbon fuel
consumption. The theory behind this concept is that the
addition of hydrogen can extend the lean operation limit,
improve the lean burn ability, and decrease burn
duration.
To apply this method to an engine a source of hydrogen
is needed. At this time the simplest option would be to
carry a tank of hydrogen. Research is being conducted
to allow the hydrogen to be reformed from the vehicles
hydrocarbon fuel supply or produce hydrogen from
electrolysis of water. In the future, better methods could
be developed for storing hydrogen in the vehicle or
production of hydrogen on-board the vehicle.
INTRODUCTION
Combustion of fossil fuels has caused serious problems
to the environment and the geopolitical climate of the
world. The main negative effects on the environment by
fossil fuel combustion are emissions of NOx, CO, CO2,
and unburned hydrocarbons. The main negative effect
of burning fossil fuel on the geopolitical climate is the
lack in supply of these fuels and the effect pollution has
on politics.
There are several possible solutions to alleviate the
problems of using fossil fuels, but most of them would
require years of further development and additional
infrastructure. This paper will look at a method of
improving fossil fuel combustion that could be
implemented without a large investment. This method
involves burning hydrogen gas along with hydrocarbon
fuels in engines.
THEORY
In order to properly understand the effect of adding
hydrogen to enrich hydrocarbon combustion it is
important to understand the basics of how an internal
combustion cycle works. Figure 1 shows a diagram of a
four-stroke engine.
Figure 1: Four Stroke Internal Combustion Engine [1]
The four stroke engine cycle is made up of four phases:
induction, compression, power, exhaust.
Induction: In the induction stroke the intake valve opens
and the piston moves down pulling in air (diesel) or
air/fuel (spark).
Compression: In the compression stroke both valves
close and the piston moves up compressing the air
mixture. Near the top of the stroke the spark plug fires
or fuel is injected (diesel).
In the power stroke the fuel burns causing the pressure
and temperature to rise driving the piston downward.
This generates the rotation of the crankshaft. At the end
of the power stroke the exhaust valve opens.
In the exhaust stroke the rotation of the crankshaft
causes the piston to move up pushing the exhaust out
the open exhaust valve. At the end of the exhaust
stroke the exhaust valve closes. The cycle is then ready
to repeat [1].
In the experimentation and performance analysis of an
engine several parameters are needed to quantify the
results. Some of these parameters include air/fuel ratio,
equivalence ratio, power, thermal efficiency, fuel
consumption and emissions.
The emissions of an engine are determined by the
operating conditions of the engine. The main emissions
of an engine are nitrogen oxidizes, carbon oxides and
unburned hydrocarbons.
NOx - Nitrogen oxidizes are generally formed at high
temperatures when N2 is oxidized in air. This reaction is
governed by the Zeldovich Mechanism, see figure 2.
Typically, if combustion temperature and residence time
are minimized and the appropriate amount of air is used
NO emissions will be small [2].
Figure 2: Zeldovich Mechanism [2]
CO and CO2 - Carbon dioxide is the product of complete
combustion. If sufficient oxygen is present then CO will
be oxidized to CO2. Carbon monoxide emissions are
more likely to occur during rich mixture conditions [2].
Unburned Hydrocarbon – Unburned hydrocarbons are
created due to several conditions in the combustion
process. In regions of the flame near the surfaces of the
combustion chamber the heat lost through the chamber
wall is greater than the heated needed to sustain a
flame. This condition causes areas of quenched flame
where hydrocarbons are left unburned.
The combustion chamber often has gaps and crevices
that do not allow a flame to propagate. These areas
allow for buildup of unburned Hydrocarbons.
During conditions where the engine is operating at partload
and reduced speed combustion is often incomplete.
This is because the combustion velocity is too slow to
allow the entire combustion charge to burn before it is
expelled during the exhaust stroke. To prevent unburned
hydrocarbon emissions, lean mixture conditions and
extended residence time at high temperature are
beneficial [2].
HYDROGEN
Hydrogen holds significant promise as a supplemental
fuel to improve the performance and emissions of spark
ignited and compression ignited engines. Appendix A
shows the properties of hydrogen in comparison to
methane and gasoline.
Hydrogen has the ability to burn at extremely lean
equivalence rations. Hydrogen will burn at mixtures
seven times leaner than gasoline and five times leaner
than methane [3]. This lower limit is governed by the Le
Chatelier Principle [4].
The flame velocity of hydrogen is much faster than other
fuels allowing oxidation with less heat transfer to the
surroundings. This improves thermal efficiencies.
Efficiencies are also improved because hydrogen has a
very small gap quenching distance allowing fuel to burn
more completely. The only drawback to hydrogen is that
even though its lower heat value is greater than other
hydrocarbon fuels it is less dense therefore a volume of
hydrogen contains less energy [3,5].
APPLICATION
The setup for introducing hydrogen into the engine
involves a hydrogen source, tanks or on-board
processor and metering equipment to measure various
combustion parameters. This process can be applied to
both spark ignited engines and compression engines.
Figure 3: Sample Setup [6]
SPARK IGNITION - Spark ignited engines can be either
fueled by liquid fuels or gaseous fuels. Propane and
methane are the gaseous fuels and gasoline and ethanol
are the liquid fuels commonly used. It can be seen in
Appendix A that gaseous fuels and liquid fuels have
different properties and react differently to hydrogen
addition, but both still benefit from hydrogen addition.
Various methods have been used to introduce hydrogen
into the engine. In one study, hydrogen was mixed with
air and compressed in a cylinder before introduction into
the engine [7]. In studies using gaseous fuels hydrogen
flow rate is matched with the primary fuel in-order to
achieve the desired percentage of hydrogen enrichment
[8]. The ultimate design for hydrogen introduction into
an engine would be using a computer control system
that would vary hydrogen percentage, equivalence ratio
and throttle with the vehicles gas pedal for optimal
running conditions [9].
COMPRESSION IGNITION - Compression Ignition
engines can be fueled with standard diesel, biodiesel or
straight vegetable oil. These engines have two options
for introducing hydrogen into the combustion process.
Hydrogen can be inducted with air into the intake
manifold or it can be directly injected into the cylinder
similar to the diesel fuel [10].
HYDROGEN ENRICHED COMBUSTION
Thermal efficiency generally is increased with the
introduction of hydrogen into an engine but it must be
properly tuned in-order to gain these benefits. Results
also seem to vary depending on the fuel used. A
properly tuned compression engine will increase in
thermal efficiency at high loads for hydrogen mass
percent of about 8% [6]. For an engine to have optimal
thermal efficiency the timing must be retarded to account
for hydrogen fast burn velocity [11]. Thermal efficiency
is related to fuel consumption with the addition of
hydrogen in all of the studies fuel consumption
decreased [6].
Figure 4 shows the effect of hydrogen addition on the
efficiency of a compression ignition engine. This figure
shows the effect of hydrogen addition on thermal
efficiency at two different load settings for diesel fuel and
jatropha oil. Figure 5 shows the effect of hydrogen
addition on reduction in fuel consumption. This figure
shows that with increased hydrogen addition fuel
consumption decreases.
Hydrogen addition gives the engine the ability to be
operated in the very lean mixture region. Lean mixtures
allow for complete combustion decreasing carbon
monoxide emissions [8].
Unburned hydrocarbon emissions are reduced because
hydrogen allows lean mixtures. They are also reduced
because high flame velocity and small quenching
distance of hydrogen promote complete combustion [12].
Figures 6 and 7 show that hydrogen addition allows the
engine to be operated with lean mixtures which reduce
CO and hydrocarbon emissions.
The addition of hydrogen increases combustion
temperatures therefore creating conditions where it is
easier for NOx to form if proper tuning is not utilized.
Several studies have shown that if mixtures are made
lean and spark timing is retarded NOx can be reduced to
a point below normal hydrocarbon combustion [2,10,11].
Figure 9 shows the reduction of NOx with increasing lean
mixtures.
Figures 8 and 9 compare the effect on emissions of the
addition of 70 g/h of hydrogen to an engine running at
3000 RPM. For these figures it can be seen that
emissions are significantly reduced by the addition of
hydrogen.
Figure 4: Variation of thermal efficiency with hydrogen
mass share [6]
Figure 5: Reduction in fuel consumption with increase in
hydrogen addition [3]
Figure 6: Effect of H2 addition on jatropha oil combustion
CO emissions [6]
Figure 7: Effect of H2 addition on natural gas
combustion hydrocarbon emissions [8]
Figure 8: Emission before hydrogen addition [2]
Figure 9: Emission after hydrogen addition [2]
HYDROGEN PRODUCTION
The greatest limitations to using hydrogen for fuel
enrichment are the cost and difficulty of production and
storage. Currently, there are three main options for
producing hydrogen for fuel enrichment. These options
are:
• Producing hydrogen from electrolysis or
reforming at a fixed location and compressing
the hydrogen for storage on the vehicle in tanks.
• Producing hydrogen, on-board by a fuel
reformer and introducing the hydrogen as
needed from the vehicles hydrocarbon fuel
supply.
• Producing hydrogen, on-board, from water using
electrolysis powered by the vehicles alternator.
COMPRESSED HYDROGEN – Compressed hydrogen
is probably the simplest but most costly method for using
hydrogen for fuel enrichment. The hydrogen must be
generated off the vehicle, compressed and stored on the
vehicle. All these steps require a great deal of cost.
FUEL REFORMER – A fuel reformer uses plasmatrons,
electrical gas heaters, which use the conductivity of
gases at high temperature to convert the liquid fuel to a
hydrogen-rich gas. Plasmatron fuel reformers have
been shown to increase engine efficiency by as much as
35% [13]. Figure 9 shows a sample diagram of a
plasmatron fuel reformer. Table 1 shows typical
operating parameters of a low current plasmatron fuel
reformer.
Table 1: Operating Parameters of a Low current
plasmatron fuel reformer [14]
Figure 10: Low current plasmatron fuel reformer [14]
WATER ELECTROLYSIS – Water electrolysis is the
process of passing an electric current through water
breaking the bonds of the water molecule to produce
hydrogen and oxygen gases. Water electrolysis
products have been shown to have a significant
improvement on performance and emission when
compared to adding hydrogen alone.
The University of Windsor in Canada conducted a
simulation of adding electrolysis products of 2 parts
hydrogen to 1 part oxygen. This study used the
CHEMKIN kinetic simulation software. It was found that
adding 10% hydrogen and oxygen was equivalent to
adding 20% hydrogen alone in reduction of emission and
improvement of performance [15].
In a second study, conducted by the University of Winsor
an experimental method was developed to simulate the
effects of adding electrolysis products to an engine. Due
to the danger of compressing a mixture of hydrogen and
oxygen, a tank with 98% air, 2% hydrogen and 1%
oxygen was prepared. This mixture was then used to
test the effect of electrolysis products. It was found that
this mixture gave the same benefits of hydrogen alone
and that the oxygen did not affect performance [16].
Performance of a commercial electrolysis unit that
produces 6.7 ml/s at 169 W was assumed as the
baseline for on-board hydrogen production [7]. It was
assumed that electrolysis products contain only
hydrogen and oxygen, no radicals. Also, no timing
adjustments were made in this setup to optimize
performance. This study concluded that an electrolysis
unit would not provide enough performance increase to
offset the energy required to run the electrolysis process.
Figure 10 shows the University of Windsor estimation of
the ability of an electrolysis unit to provide hydrogen to
enrich combustion [16].
Figure 11: The power requirement for electrolysis and
the power gained from the engine through hydrogen
addition (U of Windsor) [16]
In contrast to the findings of the University of Windsor
study, Kocaeli University replicated a patent for an
electrolysis system to provide hydrogen and oxygen for
combustion enrichment. This unit is made of a
cylindrical carbon cathode surrounding a platinum rod
anode. This unit was supplied 90 volts at 3 A and
produced about 20 L/h [17]. Figure 12 shows a diagram
of Kocaeli University’s hydrogen enrichment setup.
Table 2 shows the specifications of their electrolysis unit.
Figure 12: Electrolysis unit from Kocaeli University [17]
Table 2: Technical Specification for the Kocaeli
Electrolysis Unit [17]
This system was tested on four vehicles, fuel
consumption and emissions were measured. Table 3
displays the results for this test. It was shown that using
this system to add electrolysis products to the engine
increased fuel economy by 25-40%. The emissions, for
these cars, were also tested and were reduced from
between 40-50% depending on engine type. There was
no noticeable reduction in the performance of these
vehicles.
Kocaeli University Electrolysis test results
Vehicle % Increase Fuel Economy
1993 Volvo 940 42.9%
1996 Mercedes 280 36.4%
1992 Fiat Kartal 26.3%
1992 Fiat Dogan 33.3%
Table 3: Kocaeli University electrolysis test results [17]
From these two studies there appears to be a
discrepancy between the theoretical modeled results
and actual experiments conducted with electrolysis units
in vehicles. Either the experiments were not conducted
accurately or there is an error in the assumptions of the
models for water electrolysis products effect on engines.
Further testing should be conducted on actual
electrolysis units in engines.
Although some is not highly scientific, there is a large
amount of performance data on electrolytic gas
production and its effects on engine performance,
particularly improvement in fuel consumption from
“backyard engineers”. Searches on YouTube and
Yahoo Groups such as “Hydroxy” and “Watercar” reveal
that many people have installed water electrolysis units,
better known as “hydrogen boosters”, on their vehicles
and are achieving fuel consumption improvements
ranging from 25% to 40%. Several high performance
units are even achieving results as high as 50% to
100%. There needs to be a complete investigation of
these claims and this technology before discounting
these results.
Theories on efficient electrolysis – During the 1970’s
professor and inventor Yull Brown designed an
electrolysis power torch for use in welding operations.
This electrolysis unit was designed to pass the molecular
hydrogen and oxygen output through an electric arc.
Brown states in his patents that the electric arc splits the
molecular hydrogen and oxygen into atomic hydrogen
and oxygen radicals. When these atomic radicals are
combusted there purportedly is an additional 218,000 cal
per gram mole released than is normally assumed to be
released when molecular hydrogen and oxygen are
combusted [18].
Figure 13: Yull Brown's electrolysis unit from US Patent:
4,081,656 [18]
Figure 14: Hydrogen passing through arc [18]
Electrolytic gas, often referred to as “Brown’s Gas”, has
several interesting properties that have been observed
and utilized. “Brown’s Gas” can be generated without
the need to separate the hydrogen and oxygen
therefore, the electrical resistance between the anode
and cathode can be minimized and electrolytic velocity
can be maximized. Another interesting effect of
combustion of the gas is that it burns implosively. This
implosive burning is likely due to atomic hydrogen and
oxygen being present. Studies suggest that implosion
will only occur when there is less than 5% air in the
mixture otherwise explosion occurs [19].
When radical atoms of hydrogen and oxygen are bonded
they form what is called crystallizing π-bonds. These π-
bonds generate π-far infrared rays. These π-far infrared
rays create a strong gravitational cavity that causes the
substances to focus inward when burning. It has been
observed that this effect produces a burning temperature
of Brown’s Gas in the range of more than 6000 °C while
normal hydrogen’s burning temperature is in the range of
2700 °C [19].
Brown’s Gas has been used for many unique processes.
It can be used for welding metals and ceramics.
Brown’s gas incinerators are used to burn atomic waste
the radioactive rays are reduce to 1/3 - 1/20 of their
original strength [19].
In Korea, a Brown’s Gas incinerator is used for
vitrification of municipal solid waste.
Figure 15: Brown's Gas incinerator for vitrification of
solid waste [20]
This unit uses 6 Brown’s gas burners to melt heavy
metal ash. This unit melts flying ash and glass cullet at
a temperature of 1450 °C. This unit was used to reduce
the leachable concentration of heavy metals below the
Korean regulatory limit. The electrolysis unit for this
application is made by the E&E Company and can
produce 300 cubic meters per hour of Brown’s Gas at
600 KWH.
From the experimental data of electrolysis units and their
application for use to enrich hydrocarbon fuels and the
information about Brown’s Gas applications. It is clear
that electrolysis products cannot be assumed to be just
molecular hydrogen and oxygen. Further research and
experimentation must be conducted to determine if
electrolysis products can be used to enrich Combustion.