19-02-2013, 09:31 AM
Development and test of a new catalytic converter for natural gas fuelled engine
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Abstract.
This paper presents characteristics of a new catalytic converter (catco)
to be used for natural gas fuelled engine. The catco were developed based on catalyst
materials consisting of metal oxides such as titanium dioxide (TiO2) and cobalt
oxide (CoO) with wire mesh substrate. Both of the catalyst materials (such as TiO2
and CoO) are inexpensive in comparison with conventional catalysts (noble metals)
such as palladium or platinum. In addition, the noble metals such as platinum
group metals are now identified as human health risk due to their rapid emissions
in the environment from various resources like conventional catalytic converter,
jewelers and other medical usages. It can be mentioned that the TiO2/CoO based
catalytic converter and a new natural gas engine such as compressed natural gas
(CNG) direct injection (DI) engine were developed under a research collaboration
program. The original engine manufacture catalytic conveter (OEM catco) was
tested for comparison purposes. The OEM catco was based on noble metal catalyst
with honeycomb ceramic substrate. It is experimentally found that the conversion
efficiencies of TiO2/CoO based catalytic converter are 93%, 89% and 82% for
NOx, CO and HC emissions respectively. It is calculated that the TiO2/CoO based
catalytic converter reduces 24%, 41% and 40% higher NOx, CO and HC emissions
in comparison to OEM catco respectively. The objective of this paper is to
develop a low-cost three way catalytic converter to be used with the newly developed
CNG-DI engine. Detailed review on catalytic converter, low-cost catalytic
converter development characteristics and CNGDI engine test results have been
presented with discussions.
Introduction
Conventional natural gas engines are facing problem with in-cylinder combustion as they
produce higher unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. The NOx
emission either increases or decreases, as it strongly associated with in cylinder fuel combustion
temperature characteristics that depend on air–fuel ratio and fuel injection system (Scott
et al 2004). However, the gaseous pollutants from engine exhaust can be reduced either by
thermal or catalytic system. In order to oxidise HC and CO gases using thermal system, a
residence time of greater than 50 ms and temperature excess of 600◦C to 700◦C are required
(Heywood 1989). Temperature high enough for some homogeneous thermal oxidation can
be obtained by spark retarded (with some loss in efficiency) and insulation of the exhaust
ports and manifold. The residence time can be increased by increasing the exhaust manifold
volume to form a thermal reactor. However, this approach has limited application (Heywood
1989). Hence, the catalytic converter will be the easiest way to reduce exhaust pollutants.
Catalyst and substrate preparation
Material selection for catalyst
In this study, several stock solutions with different aqueous molar ratios and weight ratios were
used. Titanium dioxide and cobalt oxide were used as a metal oxide catalyst. The pure cobalt
oxide is used as the reducing agent and titanium dioxide is the oxidizing agent. Its inertness
to sulphate formation and surface properties makes it preferred carrier in selective catalytic
reduction of NOx from the stationary pollution sources.
Catalyst slurry preparation
Sodium silicate solution and sodium metabisulphate were used in wash coat material to
increase the coating strength to surface of woven stainless steel substrate. Ninety grams of
sodium silicate solution was added into 10·0 gmTiO2 to get 10% TiO2 slurry. The slurry was
then stirred at 500 rpm for two hours. Two grams of CoO and 1·0 gmof sodium metabisulphate
were gradually added. To ensure homogenization, it was milled for around 6·0 hours by using
ball mill at 1400 rpm and then dried frozen at temperature 23◦C for 24 hours. Slurry reactor
preparation was done as suggested by (Nijhuis et al 2001) and compared with monolithic
reactor as described in (Avila et al 2005) and (Heber 1991). The figure 1(a) shows the prepared
catalyst slurry.
Material selection for substrate
The substrate material is stainless steel, as it is widely used in the automotive exhaust system
not only due to its advantages in mechanical and physical properties but also low-cost (Bode
et al 1996). The stainless steel wire mesh was cut to a circular shape with a diameter of 7·0 cm
prior to catalyst coating. Figure 1(b) shows the wire mesh substrates.
Treatment of wire mesh substrate
The wire mesh substrates were immersed into a preparation of 10% HCl solution for
30 minutes to remove all the impurities. It was then rinsed in distilled water before being
dried in an oven at temperature of 100◦C. The drying process takes about 1·0 h before coating
it with catalyst.
Wash coat material
Titanium dioxide, TiO2 served dual functions: a reduction catalyst and a titanium substance
for the wash coat. Rutile form of TiO2 was chosen because of its thermal stability from 500◦C
and high durability. This property is suitable for catalyst embedment or catalyst as described
in (Alois 1995).
Catalytic converter test on a CNG–DI engine
Both of the catalytic converters such as conventional catalytic converter based on Pt/Rh and
new catalytic converter based on CoO/TiO2 catalyst were tested at Engine and Fuel Testing
Laboratory, Department of Mechanical Engineering, University of Malaya. The catalytic
converter fabrication was done at University Technology of Mara, Malaysia. The test engine
was a multi-cylinder compressed direct injection compressed natural gas engine. It can be
mentioned that both the catalytic converter and CNG-DI engine were developed under a
research collaboration program. The CNG-DI engine has been developed from modification
of a gasoline engine. The major modifications of the engine are: (a) changing compression
ratio from 10 to 14, (b) changing gasoline fuel injection to direct injection compressed natural
gas system, © high energy spark plug instead of normal spark plug. The displacement of the
engine is 1597 cm3. The bore and stroke are 76mm and 88mm respectively. The maximum
brake power of the engine was achieved as 73 kW at 6000 rpm. The details specification of
the engine can be seen in (Kalam 2007).
Results and discussion
Table 2 shows the legend description used in the various figures. Figure 8 showsNOx emission
versus engine speed from 1500 rpm to 6000 rpm with and without catalytic converters. The
test was conducted at wide open throttle (WOT). It is found that wire mesh catalytic converter
produces lower level of NOx emission with an average of 57 ppm all over the engine speed
range followed by OEM catalytic converter (average 255 ppm). Without catalytic converter,
the natural gas engine produces average 813 ppm NOx all over the engine speed range.
Conclusions
The following conclusions may be drawn from the present study.
• CoO/TiO2 catalyst and wire mesh based substrate based catalytic converter has been
successfully developed. The surface area of wire mesh substrate is about 25 times higher
than ceramic substrate.
• The NOx conversion efficiency of OEM and wire mesh catalytic converters are 69% and
93% respectively. Wire mesh reduces 24% higher than OEM catalytic converter.
• The CO conversion efficiency of OEM and wire mesh catalytic converters are 48% and
89% respectively. Wire mesh reduces 41% higher than OEM catalytic converter.
• The HC conversion efficiency of OEM and wire mesh catalytic converters are 42% and
82% respectively. Wire mesh reduces 40% higher than OEM catalytic converter.
• Similar reductions (as WOT) of HC, CO and NOx are found at 50% throttle and 50Nm
load conditions.
• The wire mesh catco reduces average 3·48% brake power as compared to without catco
(table A1 in Appendix A).
• Light off temperature for OEM andWire mesh Catco(s) are 220◦C to 280◦C) and 270◦C
to 360◦C respectively (figure A1 in Appendix A).