The effect of Exhaust Gas Recirculation (EGR)
on the emission from a lean-burner gas engine
Projektrapport
March, 1998
Dansk Gasteknisk Center a/s • D r. Neergaards Vej SB • 2970 Hørsholm • Tlf. 2016 9600 • Fax 4516 1199 • www.dgc.dk • dgc@dgc.dk
The effect of Exhaust Gas
Recirculation (EGR) on the emission
from a lean-bum gas engine
Per Pedersen
Danish Gas Technology Centre a/s
Hørsholm 1998
TiLle
The effect of Exhaust Gas Recirculation (EGR) on the emission from a lean-bum gas engine
Re port
Category
Project Report
Author
Per Pedersen
Date of issue
March 1998
Copyright
Danish Gas Technology Centre a/s
FileNumber
717.65; H:\717\65\rapport\EGR report.doc
Project Name
Experimental methods for reduction of emission from gas engines
ISBN
87-7795-130-1
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("General Conditions for Consulting Services (ABR 89)"), which are considered adoptedfor the assignmenJ.
•
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This English translation is providedfor convenience only and in case of discrepancy the Danish wording shall be
applicable.
July 1997
DGC-report
Table of Contents
1
Page
l Introduetion .... .............................. .............. ........ ........................... ......... ........................... ....... .... 2
2 Summary .. ....... ....... ..................................................................... .......... ..................... ..... ... .. ... ..... 3
3 En gine, generator and control unit ............................................... .... .......... ..... ............................. 5
3.1 Engine ...................................... ... ...... .......................................... ....... .. ............................. ..... 5
3.2 Generator ...................................... ... ...... .. ...... .......................... ... ............ ......... ...... ..... .... ... .... 6
3.3 Control unit .............................. ........................................................ .. ......... ... .. ........ ... ........... 6
3.4 Operation ......................................................... ............. ... ................ ..................... ... .............. 7
4 Meters, analysers and data acquisition .... ... .. ................................................................................ 8
4.1 Uncertainty ....................................... ............................................................................ ......... 8
5 Planning the experiments .... .. ... ... .. .. .. .. ... .. .. .. .. .. .. ... .. .. ... .. ... .. .. .. .. ... .. .. ... . .. .. .. . .. .. .. ... .. .. ...... ... .. .. ... .. l O
5 .l Range of parameters .. .. .. .. .. .. .. ... .. .. .. ... .. .. .. .. .. .. ... .. .. . .. .. .. .. .. .... ... .. .. ... ..... .. ... ...... ... . .. ... .. .. .. .. .. . .. l O
5.2 Succession of measurements ....................................... ................................. .............. .. ....... lO
6 Measurements ........................................................ .. .... ...... ..... ... .... .. ...... ...... .............. ....... ....... .. 12
6.1 Ignition timing and air/fuel ratio without EGR .................................................................. . 12
6.2 Measurements with EGR ....... ... ....................................................................................... .... 15
7 Closing remarks .......................................... .... .................. ... ...... .. ...... ........ ... ................. .......... .. 18
Appendix ....... ... ........ ... ....... ............ .... .......... .. ............. ................. ....... .......... .............................. .. 19
Analysers .................... .................. ... ..... .... ..... ... ... ......... ........... ......... ...... ....... ................... ............. 19
Oz ........................................ ...... .. ....... ........... .................................................................... ........ . 19
NOx········· ······································ ················· ····· ······ ············································ ············ ···· ····· 19
co .............................................................................................................................................. I9
THC ........... ........... ... .... .......... .. ............................................................................. ............. ........ 20
COz .... .......... ... .............. ..... ...... .. .. ..................... .............. .. ........ ... ...... ........ ...... ... ... .............. ..... . 20
Literature .......... .. ............. ............. .............. .......................................................... ..... .......... .... ...... 21
DGC-report
2
1
Introduetion
In recent years, attention has been focused on the emission of unbumed hydrocarbons in the exhaust gases from natura} gas engines. The stationary gas
engines instaHed in Denmarkare almost exclusively operatedasparts of cogeneration plants. The composition of unburned hydrocarbons is dominated
by methane, which is a greenhouse gas.
Emission of total unbumed hydrocarbons (THC) can be caused by many
factors, such as crevice volumes in the combustion chamber and overlapping
valves. Another factor is the mixture completeness. Running an engine on a
Iean mixture is an effective way of reducing nitric oxides in the exhaust
gases. The combustion temperature becomes lower leading to lower formation of nitric oxides (Zeldovich). Unfortunately, running anengine on a Iean
mixture has often an adverse effect on the emission of unburned hydrocarbons.
Several solutions such as catalysts and improved combustion are presently
being investigated. This project investigates the effect of using exhaust gas
recirculated to the air intake as replacement for air excess.
The work has been sponsored by the Danish gas companies and carried out
at the labaratory at Danish Gas Technology Centre a/s (DGC). Q/A on this
report was done by Brian Schmidt, DGC.
Hørsholm, March 1998
f
h
J~'
Per Pedersen
Bjarne Spiegelhauer
Project Manager
Viee-President
Dept. of Gas Utilization
Dept. of Gas Utilization
DGC-report
3
2
Summary
Exhaust gas recirculation (EGR) is a well known method of centrolling
emission of nitric oxides from engines. It w as used on petrol engines befare
introduetion of the three-way catalyst. Recently, it has been re-introduced in
cernbination with the three way catalysts.
On automotive petrol engines smaller rates of EGR are utilised.
When larger rates of EGR are applied, it is known from various sources in
the literature - e.g. /2/ and /3/- that the THC content in exhaust gases increases, and, eventually, severe misfires occur at about 30% EGR. In this
project, only smaller EGR-rates were investigated.
Purpose
The project should demoostrate an indirect effect of EGR on the content of
total unburned hydrocarbons (THC) in the exhaust gases from a lean-bum
natura! gas engine.
When running a specific engine on lower air excess (without EGR), it is
known that the THC content in the exhaust gases is aften lower compared to
the THC content when running the engine on higher air excess. On the other
hand, nitric oxides will increase rapidly at decreased air excess. The formation of nitric oxides can be suppressed by the use of EGR.
A small rate of EGR has littie effect on the emission of THC according to
the literature. The indirect effect of replacing air excess with EGR should
thus be a reduction of the emission of THC.
Results
The tests showed that a small amount of about l% (EGR) reduces the nitric
oxides signifieand y, with no detectable undesired effect on fuel consumpti an or THC emission. This is the case for gas engines running moderately
lean (max. 50% air excess), and EGR can thus be recommended for these
engines. It lowers the conten t of nitric oxides, or the engine can be operated
on a richer mixture leading to reduced THC emission.
On engines running on ultra lean mixtures, EGR has littie effect, mostly
disadvantages.
The experiments reported here were carried out on a supercharged natura!
gas lean-bum engine instaBed in the labaratory at DGC. The engine has
been built into a mini co-generation unit comprising control unit, asynchronous generator and heat exchanger. Emission characteristics have been reported in an earlier project repart Il/.
DGC-report
4
The engine has since then been modified. Forthis reason, the emission characteristics were re-investigated prior to the EGR experiments.
Results without
The emission of both NOx and unburned hydrocarbons is much lower com-
EGR
pared to the earlier engine layout. On the other hand, the emission of carbon
monoxide has increased and an oxidation catalyst is required in order to
fulfil the Danish legislation for CO 15/.
From samples of exhaust gases taken just after the exhaust manifold and
samples taken after the turbocharger, it was noted, that unburned bydroearbons were oxidised in the exhaust system. The effect is pronounced at low
air excess
(A~
1.5) and retarded ignition timing. With increased air excess,
the effect becomes negligible.
Hydrocarbonsis decomposed into carbon monoxide which oxidises further
to C02, but the last step takes place at a slower rate. Thus more CO is observed in the exhaust gases after passage of the hottest part of the exhaust
system.
Results with
A small amount of cooled and dried exhaust gasses was feed back to the
EGR
induction system, after the carburettor at the suction side of the turbocharger. The amount was measured to be in the range of l% of the volume
of air consumed. This yielded almost 20% reduction in nitric oxides.
The results are similar to other EGR experiments on lean-bum engines reported in the literature, but here the results are based on larger EGR rates.
See e.g. 121 and /3/.
DGC-report
5
3
Engine, generator and control unit
The complete unit comprising engine, generator and control unit has been
designed as part of a masters thesis by two students at the Copenhagen
Technical College. The work was done in co-operation with a company
manufacturing small co-generation units. The work was sponsored by the
Danish gas companies, and DGC carried out measurements of emission and
efficiency.
After the masters thesis, the unit was moved to DGC and installed.
3.1
Engine
The engine is a former Ford diesel engine converted by Power Torque for
natural gas operation under the designation SI4. It is a four-cylinder, fourlitre dispiacement engine.
The engine was not built for lean-bum operation, therefore some modification has been made. In order to secure ignition and to maintain high power
output at a lean mixture, the ignition system has been changed and a turbocharger has been added.
The gas is mixed with air in an Impco gas carburettor. Mixture adjustment
c an be carried out manually, while the engine is running. The throttle is operated by a servo motor and a position sensor for remote control. After the
carburettor, the mixture is compressedin the turbocharger. After compression the mixture is cool ed in an intercooler (Mermaid type 4) to about 30400C.
The turbocharger is a Garret type T2 with waste-gate. The waste-gate allows
for manual adjustments of charge pressure, which makes it possible to
maintain the same power output at higher air excess.
Between the exhaust manifold and the turbocharger a reactor has been
added. It consists of a l oo conical expansion from the manifolds, Ø50 internal diameter on the flange up to 0129. Tubes of this diameter extends for
c a. l, 17 m through two rounded bendings, directing it to the turbocharger.
Anether l oo con e contracts the diameter to match the turbochargers turbine
inlets. All pipes, cones and flanges are made of stainless steel.
DGC-report
6
The reactor was added as part of another project investigating the effect of
adding a strong oxidising agent to the exhaust gases. Both the manifold and
the reactor are insulated in order topreserve a high temperature in the exhaust gases and to avoid radiant heat. It soon turned out, that the reactor was
active by itself, oxidising more or less unburned hydrocarbon.
The original ignition system has been replaced by a Motortech IQ250 capacitive discharge ignition system. The ignition timing varies with engine
RPM after a linear relation. The setting can be manually adjusted or remotely controlled (not used). The selected spark plugs were Champion R.L.
85.G recommended by Power Torque.
The progress of combustion was briefly monitored by means of a Kistier
spark plug with pressure transducer. No signs of knock were ever detected,
regardless of operating conditions.
3.2
Generator
The engine is coupled to an asynchronous generator with a nominalload of
37 kW. The electricity generatedis disposed off by feeding it into the main
electricity supply.
3.3
Control unit
The engine and the generator is controlled by a PLC which takes care of
important operation and monitering tasks, such as starting the engine (using
the original starter motor), running the engine at idle for a few minutes before increasing the speed slowly to slightly above 1500 RPM. When the engine has reached this speed, the generator is coupled online. Then the power
is increased slowly until output power reaches 37 kW. The PLC program
then maintains this load, while monitering coolant temperature and lubrication oil pressure during operation. If certain limits are reached, shut-down is
automatically initiated. At normal shut-down procedure, the load is slowly
reduced to O kW, then the generator is de-coupled and the enginespeed is
slowly reduced to idle speed, at which the engine is kept running for five
minutes in order to cool the turbocharger before stopping. Other conditions
will cause emergency shut-down.
DGC-report
7
The engine and the generator are encJosed in a noise-insulation housing. A
door at one end of tbe hou ing provides access to the en gine. At the opposite
side of the housing, the control unit is placed. The conu·oJ unit has two
witches, a knob and an emergency stop button. A small display indicates
the engineload at normaJ operation. At error conditions, the display indieates the error, e.g. missing oiJ pressure.
3.4
Operation
When normal engine load has been reached, the operator can manually override the part of the PLC-program centrolling the engine load, by turning a
switch into the MANUAL position and then, by turning a knob, either decrease or increase load until full throttle. Befare shutting down the engine,
the switch must be turned back into the AUTO position. Then the first
switch can be turned into the STOP position, and the shut-down procedure
starts. An emergency stop button is placed in the centre of the front panel.
All experiments were carried out in the MANUAL position, and the power
output was adjusted to 35 kW in every measurement.
DGC-report
8
4
Meters, analysers and data acquisition
During installation in the labaratory at DGC, the engine has been equipped
with a number of sensors and meters connected to a data acquisition system.
The sensors and meters are shown in Pigure l.
A number of measured values are instantly processed and shown on the dis-
play. This is the case for actual 0 2 and C0 2 in the exhaust gases and emission of NOx (NO and N0 2), CO and THC. Purther values shown are fuel
consumption, power and heat produetion and the efficiency of electricity
production. The acquisition system scans all chanoels at an interval of two
seconds. Measured and calculated values are stored in files.
Values of ignition timing must be read using a stroboscope and noted manually.
Fuel - pressure and temperature
Volume flow olluel
Intet manifold- pressure
and temperature
lntercooler
Temperature ol exhaust gases
belore and alter turbocharger
Spark plug No l
replaced by a
Kistier spark plu
whh pressure
transducer
Generator
Temperature ol
lubrtcation oll
F ig ure l: Location of the various meters and sensors
4.1
Uncertainty
The various meters and analysers contribute to the overall uncertainty on the
resulting values of emission and efficiency. The foliowing tab le lists the
tolerance of the meters.
DGC-report
9
Min.
Max.
Values during
Tolerance
experiments
Gas meter
1 m"/h
16
Flow meter
0,05 m"/h
Electricity
Pressure
m~/h
10 m"/h
±1%.
2,5 m"/h
O, 1 m"/h
±3%.
O kW
50 kW
35 kW
±0,5%
O bar
1 bar
0,3 bar
±0,6 hPa
Barometer
O bar
2 bar
1013 hPa
±0,5 hPa at 1 bar
Temperature
NiCr/Ni Al
100°C
1100°C
680°C (exhaust)
±3,?DC at
Temperature
Pt 100
ooc
400°C
20°C (gas)
±0,3°C
coolant flow
manifold and gas
soooc
35°C (intake
manifold)
The gas analysers (Appendix l) were calibrated each day using test gases of
±2% relative uncertainty on the concentration. Prior to each measurement,
the analysers' ranges were changed if necessary, and se al e factors in the data
acquisition program were changed accordingly.
Analyser
Measure d
Relative uncertainty [%]
02
5,5 vol%
±4,7
co2
8vol%
±2,7
CO
1000 ppm
±3,5
NO x
700 ppm
±4,0
THC
150 ppm
±2,7
DGC-report
1O
5 Planning the experiments
Previously, many measurements on stationary engineshave been carried out
maintaining certain conditions and then varied e.g. load, air excess, speed or
EGR.
In the literature these conditions are abbreviated as e.g.
WOT
Wide Open Throttle
BPSA
Best Performance Spark Advance
MBT
Minimum advance for Best Torque
W e decided to carry out the measurements at a fixed load of 35 kW at various values of ignition timing and air excess. First, we measured without
EGR in a wide range of air excess and a more confined range of ignition
timing. The EGR w as applied and some measurements (at lo w air excess)
were repeated.
5.1
Range of parameters
According to the manual, the SI4 engine should be running at an ignition
timing at of 17° BTDC, when running on rich mixture. A lean mixture bums
at a slower rate, so higher ignition timings were considered to be of interest.
The various experimental set points were selected as 18, 20, 22 and some at
24°BTDC.
From earlier measurements it was found that a compromise between NOx
and CO was found at 50% air excess, A= 1,5. A range of A from 1,3 to 1,8
was selected.
Measurements with EGR were only carried out at the lowest air excess ratios.
5.2
Succession of measurements
Ideally, the measurements should be situated equidistantly within a matrix.
For elimination of systematic errors, the succession should be in random
order.
DGC-report
11
In practice, it is rather time consuming to adj ust air/fueJ ratio and subsequently engine power. Therefore, some rneasurements were carried out in
systematic succession. At fixed air/fuel ratio, the ignition timing, wbich is
much easier to handle, and subsequently engine power, were adjusted.
DGC-report
12
6
Measurements
Bach measurement takes approx. an hour. First, the engine is adjusted. Then
after a while, the engine has settled and the analysers read out more or Jess
constant values. Final adjustments of power output and air/fuel ratio must
then be carried out. After a while, the engine has settled again. Then the
actual measurements can take place. This willlast about 10 minutes. The
acquisition system scans all channels at a two-second interval, until 500
rows of data have been collected, then the program stops.
At constant load, a large amount of measurements was carried out within in
a matrix of air/fuel ratio and ignition timing as deseribed in section 5.1.
Measurements were carried out at both the manifold and in the stack.
6.1
lgnition timing and air/fuel ratio without EGR
The ignition timing compensates for the burning velocity: if the engine runs
slowly, the charge maybeable to bum completely at a given ignition setting.
When the enginespeed is increased, the ignition must be advanced in arder
to allow the charge to be burned befare the exhaust stroke starts. Furthermore, the burning velacity is dependant on the air/fuel ratio. Atleaner mixtures, the charge bums at a slower rate. Finally, the flame paths and combustion rates vary with the combustion chamber design and dimensions.
lgnition timing (the time when the spark is fired) is defined as degrees of
crank angle befare top dead centre ( BTDC).
0
DG C- repart
13
ppm
ppm
21
02 [vol %]
lgnition timing [ BTDC]
0
18
5
Pigure 2: CO measuredin samples taken in the exhaust manifold
ppm
ppm
02 [vol %]
lgnition timing [ 0 BTDC]
18
5
Pigure 3: NOx measuredin samples taken in the exhaust manifold
DGC-report
14
..
-
~
.
.
~.
.
THC [ppm]
THC [ppm]
..
·,
•
•
-
l
Oxygen [val%]
lgnition ["BTDC]
18
5
Pigure 4: THC measured in samples taken in both the exhaust
manifold and in the stack
By looking at Figure 3 and Figure 4 it can be seen that when running at low
air excess, the engine produces unacceptable high levels of nitric oxides,
while the level of unburned is very low, especially in the stack. If nitric exides could be reduced without affecting the level of THC, the engine could
be run at lower air excess. This would yield an indirect effect of reducing
THC.
The particular engine used for the experiments hereis scrnewhat special, as
it has a large highly insulated volume just after the exhaust manifold - the
"reactor". The volume is about four times the swept volurne of the engine. It
is known from literature /4/, that hydrocarbon can be burned in the exhaust
at temperatures above 650°C and at relatively long residence time. As the
exhaust temperature from the engine used for this project reaches 680°C,
when the reactor is effective, some of the hydrocarbon will be oxidised,
which appears from the measurements shown in Figure 4. There may be
more complex reactions and this is being studied in anether project at DGC.
For commercial engines, a level of THC corresponding to the level marked
"Manifold" in Figure 4, or slightly lower, can be expected.
DGC-report
15
The level of carbon monoxides (see Pigure 2) is high, and as mentianed earlier, there iseven more when measuredin the stack, due to the reactor. This
is no problem, as oxidation catalysts are effective in removing carbon monoxide.
6.2
Measurements with EGR
If Pigure 4 is viewed from the right side, it willlook as follows:
10000
ec.
.E!;
o
1000
..
100
:t:
1-
10
•~ ~ ;
~
,
...
.
... ....
•
~
• THC without
EGA
•
5
6
7
8
9
10
11
0 2 [vol%]
Figure 5: THC versus 0 2 in exhaust gases. As many different ignition
timings are represented, the data is sarnewhat scattered.
It is seen that a significant increase in THC eecurs at increased air excess.
It was decided to apply EGR at air excess corresponding to 5.5% 0 2 .
Exhaust gas was lead though a coil providing cooling to at bottle calleeting
condensed water. The dry gas was measuredin a flowmeter with a full scale
of 65 litre/minute.
Measurements were then carried out at several ignition timings, and the rate
of recirculated gas was kept constant at l% based on valurnes at normal
condition.
16
DGC-report
1000
900
e
Q.
B
d
z
800
+ NOx w ithout EGA
700
o NOx w ith EGA
>.
600
500
5.50
5.52
5.54
5.56
5.58
5.60
0 2 [vol%]
Figure 6: 18° BTDC
1000
•
900
'[
800
~
d
z
•
• NOx w ithout EGA.
.....
o
o NOx with EGA
700
600
500
5.40
5.45
5.50
5.55
0 2 [volo/o]
Pigure 7: 20° BTDC
5.60
5.65
5.70
DGC-report
17
1300
1250
1200
1150
E
1100
c.
~ 1050
& 1000
z 950
900
850
800
•
5.45
w ~hout EGA
<> NOx w ~h EGA
• NOx
6
...,
5.50
5.55
5.60
0 2 [vol%]
Figure 8: 22° BTDC
1400
~
1300
•
Ec.
+ NOx without EGA
S: 1200
<> NOx wilh EGA
)(
o
z
o
1100
1000
5.45
5.50
5.55
5.60
0 2 [vel%]
Figure 9: 24° BTDC
The figures show reductions in the range of 15 to 20% of nitric oxides.
No direct effect of EGR on THC was seen. No detectable difference in fuel
consumption or electrical efficiency between load with or without EGR w as
observed. It is assumed, however, that the electrical efficiency will decrease
at increased EGR and maintained 0 2 content in the exhaust gas.
DGC-report
18
7 Closing remarks
During the experiments, it became clear that the rate of EGR should be determined by other means than a flowmeter. When using flowmeters, the exhaust gases should by dried. Otherwise, the flowmeter becomes fouled by
condensate. A better way is to measure C0 2 both in exhaust gases and in
combustion air after mixing.
W e did not achieve levels of nitric oxides fulfilling the Danish legislation /5/. To do so requires higher rates of EGR and slightly higher air excess.
Our best guess is to run the particular engine at 6% 02 and to increase the
EGR rate to 5-7%. Unfortunately, there was no time to verify this.
More literature on EGR has been published since this project was initiated.
It is well documented in the literature, that a reduction of nitric oxides of
50% can be achieved by using EGR rate of about 7%. There is reported no
significant increase in THC emission at this level /2/. EGR has littie effect
on engines running on very lean mixtures, due to smaller contents of water
vapour and C02 •
Finally, attention should be paid to cerrosion and wear problems when establishing EGR on commercial natural gas engines. Water vapours condense
in the pipe leading exhaust gases to the inlet. Droplets could damage the
turbo charger and if materiais such as copper are used for EGR pipes, corrosion could present a problem to the engine.
DGC-report
19
Appendix
Analysers
The sample of flue gas was cooled, filtered and dried in an air conditioner
before being fed into the analysers.
The instruments arelabelled (DGC No.) and a log is kept for each instrument.
The analysers we re calibrated every day using test gases of ±2 o/o relative
uncertainty. The test gases used were the following. The bottles oftest gases
are replaced minimum each year.
Manufacture
Type
Servemax
572 - paramagnetic
Range
O- 25 vol. o/o
Accuracy
± 0.1 vol. o/o
DGC-no.
201
Manufacture
Thermo Environmental Instruments Ine.
Type
Range
lOA/R
from O- 2.5 to to O- 10.000 ppm
Reproducibility
l o/o of full scale
Linearity
±l%
DGC-no.
302
CO
Manufacture
Hartmann & Braun AG
Type
Range
Reproducibility
Uras 3 G
from O- 200 to O- 2.000 ppm
~
0.5% of full scale
20
DGC-report
l
Linearily
DGC-no.
±l
%1
402
THC
Manu faeture
Type
Range
Analysis Automation Ltd.
523
from O- 2.5 ppm to O- 10.000 ppm
Accuracy
± l vol. % o f range
DGC-no.
601
Manufaeture
Type
Range
Reproducibility
Hartmann & Braun AG
Uras 3K
O - l O and O - 20 vol. %
::::; 0.5% of full scale
Linearity
::::;±1%
DGC-no.
501
DGC-report
21
Uterature
Pedersen P.: Measurements of emission from a lean-burn gas engine.
Hørsholm: Danisb Gastechnology Centre, 1997.
2
Raine, R.,R.; Zhang, G.; Pflug A.: Comparison of Emissions from
Natural Gas and Gasoline Fuelled Engines. SAE paper 970743, 1997.
3
Johansen, Bengt; Docekal, Daniel: Effekt av A., EGR ocb tandvinkel på
emissioner och virkningsgrad i e n konverterad naturgasmotor. Lund:
Lunds Tekniske Højskole, 1995.
4
J., B ., Heywood: lnternal Combustion Engine Fundamentals. McGrawHill International, 1988.
5
Miljøstyrelsen: "Bekendtgørelse om kvælstofilteforurening mv. fra
gasmotorer og -turbiner", bekendtgørelse nr. 688 af 15/10-1990