Reduced NO, emissions from internal combustion engines fuelled by natural gas

Timothy Fowler British Gas plc, Watson House Research Centre, Peterborough Road, Fulham, London SW6 3HN

David Lander and Diane Broomhall British Gas plc, Midlands Research Station, Wharf Lane, Solihull, West Midlands B91 2JW, UK (Received 28 August 7990; revised 78 November 7990)

Commercial 3-way catalytic converters, developed for the automotive industry, have been applied to natural gas fuelled stationary internal combustion engines which are used in combined heat and power schemes. Emissions of nitrogen oxides (NO,) are reduced by >95% under suitable conditions, with no appreciable loss of catalyst performance after 4860 h. The combustion products were also analysed for nitrous oxide and ammonia before and after NO, reduction. The concentration of both species was found to be less than 5 vpm. Thus the selectivity of the NO, reduction reaction to nitrogen was inferred to approach 100%. Furthermore, the emissions of methane and carbon monoxide are reduced by 50 and 98% respectively, and the combustion condensate is significantly less acidic after exhaust treatment with a 3-way catalytic converter.

(Keywords: combustion; gas; nitrogen emission)

It is estimated that the major source of emissions of nitric oxide and nitrogen dioxide (collectively termed NO,) attributable to human activities is the combustion of gasoline in automobile engines’. Recent EEC legislation has aligned European emission limits with those in force in North America and Japan’. This is only achievable (at least in the immediate future) by adopting the so-called 3-way catalytic converter (TWC) which is designed to promote the reaction of partially burnt gasoline with NO, to form carbon dioxide, water and nitrogen. To achieve this the combustion air to fuel ratio must be precisely controlled at the equivalence point. This is performed by using an oxygen sensor upstream of the TWC to modify the action of the fuel injector.

Combined heat and power schemes (CHP) are commercially and environmentally attractive because of their high overall efficiency, which leads to low fuel costs and low emissions of carbon dioxide (a major greenhouse gas). Many CHP schemes use a natural gas fuelled internal combustion engine to generate electricity via an alternator. Heat recovery from the engine is the cause of the high efficiency. In common with other spark-ignited internal combustion engines, and indeed many electricity generation devices, a CHP engine emits a significant quantity of NO,. The reason for this is conceptually simple2” : The air fuel mixture is preheated by compression and by contact with the hot cylinder walls. It is then burnt under approximately constant volume conditions. Under these conditions C,, the heat capacity at constant volume, is more appropriate than C,, the heat capacity at constant pressure. For the normal conditions C, is less than C, due to the absence of pressure-volume work at constant volume. This results in an increased flame temperature compared with that obtained when the

combustion products can expand freely, and consequently increased formation of NO,. (NO, formation increases rapidly with temperature2b-2d.) This concentration of NO, is then ‘frozen’ by the rapid reduction in exhaust gas temperature that is the result of gas expansion into the relatively cool exhaust manifold when the exhaust valve opens.

The initial aim of the work described below was to reduce NO, emissions from CHP schemes by the application of TWC techniques. This in itself is novel as methane, the major component in natural gas, is the hydrocarbon that most resists oxidation. Thus it was not clear at the outset if natural gas would be a suitable reductant for NO, in a standard TWC. The work was subsequently extended to include the analysis of minor components of the exhaust stream, as some authors have suggested that NO, reduction with a TWC results in appreciable emissions of ammonia3 and nitrous oxide4v5. These analyses were performed in order to quantify the environmental acceptability of natural gas combustion and NO, reduction with TWCs, with reference to the projected problems of global warming (greenhouse effect) and the destruction of the ozone layer.

EXPERIMENTAL

The exhaust gas samples were taken from a Dovergas S16 natural gas fuelled engine fitted with a TWC supplied by Johnson Matthey (type P12CEB/50/5.1). The combus- tion air to fuel ratio was controlled with equipment developed by British Gas. This consists of a variable inlet pressure gas carburettor, the action of which is modified by a signal from an oxygen sensor upstream of the TWC. Emissions of carbon monoxide, carbon dioxide and

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Reduced NO, emissions: 1. Fowler et ai.

methane were monitored in the dried exhaust gas by on-line non-dispersive infra-red analysers supplied by Analytical Development Company. The concentrations of oxygen and NO, in the dried exhaust gas were measured on-line by a Servomex paramagnetic analyser and an Analysis Automation chemiluminescence analyser respectively. Additional measurements of the NO, concentration in wet exhaust gas, via a heated sample line, confirmed that less than 4% of the inlet NO, concentration was lost during drying of the exhaust gas. This result was independent of the NO, concentration in the wet exhaust gas over the range investigated, and is consistent with previous result@, since the NO, observed in the engine exhaust is predominantly nitric oxide.

Other analyses were performed off-line. The concen- tration of hydrogen was determined by gas chroma- tography using a column packed with Molecular Sieve 5A connected to a Thermal Conductivity Detector. A PU4850 Video Chromatography Control Centre was used for peak integration. The gas chromatographic analysis of other components in the gas sample served as a useful crosscheck of the on-line analysis. A Nicolet SBX 20 Fourier transform infra-red spectrometer fitted with a Spectra-Tech 10 m path length gas cell was used off-line to quantify the concentration of nitrous oxide in the exhaust stream. Samples for nitrous oxide analysis were collected in PTFE gas bags and loaded (within

b

30 min) into the gas cell through a soda lime and calcium sulphate absorption system to eliminate absorption peaks due to carbon dioxide and moisture. This is necessary to enable measurement of the nitrous oxide line at about 2200 cm-l, since this lies between overlapping lines due to carbon dioxide and carbon monoxide. In the complete absence of carbon dioxide it is possible to quantify the concentration of nitrous oxide by a combination of spectral addition (nitrous oxide spectra added to test spectra) and spectral subtraction (carbon monoxide spectra subtracted from test spectra) coupled with residual peak integration. This method, illustrated in Figure 1, was verified using gas mixtures of known composition. Control experiments indicated that less than 4% of the inlet concentration of nitrous oxide was absorbed on fresh absorbent, in agreement with other workers’. The ammonia concentration was determined by passing a known volume of wet exhaust gas through boric acid, followed by measurement of the ammonium concentration by an ion selective electrode. The sampling scheme is in Figure 2.

RESULTS AND DISCUSSION

Some representative on-line analysis results of the exhaust gas produced by the engine are shown in Table 1. These results indicate that about 95% of the NO, produced by the engine is converted to other species over

4000 3600 3200 2400 2000 Wavenumber

Figure 1 FT-i.r.spectra. a) Exhaust gas post catalyst with most moisture removed, b) 50vpm N,O in N, calibration gas, c) exhaust gas post catalyst minus CO, by absorption, d) exhaust gas post catalyst minus CO, by absorption minus CO by spectral subtraction

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Reduced NO, emissions: T. Fowler et al.

the TWC. This is achieved for an inlet temperature of about 470°C at an estimated gas hourly space velocity of 100000 h- ‘. This space velocity is calculated as volume of exhaust gas at stp per volume of catalyst monolith. An equivalence ratio (air to fuel basis) of 0.98 to 1.0 can be estimated from the gas analysis shown in Table 2. In addition to reducing the emissions of NO, by 95%, the TWC also reduces emissions of carbon monoxide and methane by about 98 and 50% respectively. This results in an increase in the emission

I NOx * vent

wet exhaust gas

02

drierite

t vent

v GC, FT-IR

sample

pH sample

Lr-ri on-line analysers

I

flow meter

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NH3 sample boric acid

Figure 2 Schematic diagram of sampling system used

Table 1 On-line gas analysis from the exhaust of Dovergas S16 engine expressed on a dry gas basis. Before = before TWC, After =after TWC, 0.0% = no gas detected

0, CO CO, CH, NO, NO Temp. Position (%) (%) (%) (%) (vpm) (vpm) YC)

Before 1.1 1.2 11.2 0.11 2620 2610 455 After 0.0 0.016 12.7 0.06 15 65 590

Before 1.1 1.25 10.7 0.11 2690 2690 475 After 0.0 0.012 12.2 0.05 75 75 590

of carbon dioxide and in the temperature of the exhaust stream. In effect the use of the TWC results in more complete combustion of fuel. Thus suitable positioning of a heat exchanger, downstream of the TWC, could result in a further increase in the already high overall efficiency of a CHP system. It should be noted that the results shown in Table 1 are time averaged. As the air to fuel ratio changes, as a result of the action of the controller, the NO, and carbon monoxide analyses downstream of the catalyst vary in anti-phase by up to 15% of the average analysis.

The results of off-line analyses of exhaust gas are shown in Table 2. It should be noted that the gas chromatography analysis fails to resolve oxygen and argon. Since the concentration of argon in dry air is 0.930h8 it follows that the concentration of argon in dry stoichiometric exhaust gas is about 1.0%. Moreover, the variation in this concentration with equivalence ratio is insignificant within the terms of this report. Hence when the argon contribution is subtracted from the analyses in Table 2, reasonable agreement is seen between the on- and off-line analysis methods. Similarly, there is reasonable agreement between the analyses of carbon monoxide, carbon dioxide and methane shown in Tables 1 and 2. In addition Table 2 indicates that a significant amount of hydrogen is produced by the engine, but this concentration is effectively reduced to zero over the TWC.

The results from additional experiments, shown in Table 3, were performed with a gas blender designed to produce synthetic exhaust gas of variable composition. Results in Table 3 indicate that the TWC promotes the water gas shift reaction (Equation (1)) steam reforming of methane (Equation (2)) and carbon dioxide reforming of methane (Equation (3)).

Table 3 On-line gas analyses from the gas blender expressed on a dry gas basis. Before = before TWC, After = after TWC, 0.0% = no gas detected

0, CO CO, CH, NO, Position (%) (%) (%) (%) (vpm) Steam

Before 0.0 0.15 1.0 After 0.0 0.02 1.1

Before 0.0 0.04 0.4 After 0.0 0.06 0.7

Before 0.0 0.96 8.0 After 0.0 1.08 7.9

Before 0.0 0.0 0.0 After 0.0 0.15 0.0

0.0 0.0

0.38 0.06

0.06 0.02

0.4 0.2

0.0 Present 0.0 Present

0.0 Present 0.0 Present

0.0 Absent 0.0 Absent

2900 Absent 550 Absent

Condensate

PH

4.1 6.4

4.0 7.4

Table 2 Off-line gas analyses from the exhaust of Dovergas S16 engine expressed on a dry gas basis. Before= before TWC. After =after TWC, 0.0% = no gas detected, < (detection limit)= gas detected, but not quantified

Gas chromatography

Ar+O,’ co CO* CH, HZ Position (%) (%) (%) (%) (X)

Before 2.09 1 .Ol 10.6 0.10 0.78 After 1.20 0.0 11.9 0.05 0.0

Before 2.04 0.97 10.6 0.12 0.78 After 1.22 0.0 12.0 0.06 0.0

’ Analysis by Ion Selective Electrode following absorption in boric acid b Concentration expressed on dry carbon dioxide free basis ’ Ar and 0, peaks not resolved. Concentration of Ar in exhaust about 1 .O%, see text

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NH, (vpm)

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<5 <5

<5 <5

FT-ir.”

N,G (vpm)

3 5

3 4

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Reduced NO, emissions: T. Fowler et al.

CO+H,O =CO,+H, (1)

CH,+H,O=CO+3H, (2)

CH,+CO, =2CO+2H, (3)

Thermodynamic calculations confirm that the products of these equilibria are favoured under exhaust gas conditions.

Whilst Tables 1 and 2 suggest hydrogen and carbon monoxide are the major reductants of NO,, results from the gas blender confirm that, at temperatures greater than 400°C methane will react directly with NO, (i.e. in the absence of other gases except nitrogen). The reaction products are carbon monoxide and (presumably) nitrogen and hydrogen or water, see Table 3. Thus the reduction in methane concentration over the TWC observed in Tables 1 and 2 may be due to oxidation with oxygen or NO,, or due to Equation (2) or (3). Indeed, in real exhaust gases (containing both carbon dioxide and steam) the dominant mechanism whereby unburnt fuel reduces NO, may be indirect. That is, the fuel reforms over the TWC, via steam or carbon dioxide, to give carbon monoxide and hydrogen which then reduce NO,. The existence of these equilibria illustrates the weakness of inferring the performance of TWCs from simple gas mixtures. Furthermore, since methane, carbon monoxide and hydrogen can all be shown to reduce NO, directly over the TWC, it is suggested that the relevance of some surface science results (e.g. reference 9), which concen- trate on the CO-NO reaction, may need to be re-evaluated.

Although Table 1 clearly shows that the application of TWC methods to engines fuelled by natural gas can reduce the emissions of NO,, it does not show the selectivity of the reduction reaction. Table 2 shows the results of off-line analyses of ammonia and nitrous oxide in the exhaust gas. The concentrations of both these species are at the detection limits of the techniques employed. Nevertheless the results indicate that NO, reduction within the TWC does not result in high emissions of either ammonia or nitrous oxide, although the concentration of nitrous oxide is observed to increase slightly. This was a surprising result as some authors4s5 have suggested that large concentrations of nitrous oxide (up to 180 vpm) are formed during NO, reduction with TWCs installed in the exhaust of gasoline fuelled engines. Concentrations this high were associated with catalyst inlet temperatures of about 360°C. Quoted nitrous oxide concentrations5 obtained under comparable conditions to those cited in this report are of the order of 10-100 vpm for gasoline fuelled engines, depending on the exact experimental conditions. The large difference observed between gasoline and natural gas fuelled engines may be attributable to the much lower levels of sulphur found in natural gas compared with gasoline. Recent work” suggests that nitrous oxide formation is linked to the presence of sulphur dioxide, the concentration of which

is negligible in natural gas combustion products. The implication of these results therefore is that the selectivity of the NO, reduction reaction to nitrogen over the TWC is close to 100% when the fuel is natural gas. The above results appear to be neatly summarized by the pH of the condensed combustion water. Before treatment with the TWC this is mildly acidic, whereas after treatment the pH is close to neutral.

CONCLUSIONS

Standard TWCs can be used to reduce emissions of NO, from internal combustion engines by 95% when natural gas is the fuel, provided the combustion air to fuel ratio is correctly controlled. There is a concomitant reduction in emissions of methane and carbon monoxide of 50 and 98%, and a reduction in acidity of the condensed combustion moisture. Measurements of the concentration of ammonia and nitrous oxide indicate that the selectivity of the NO, reduction reaction to nitrogen over the TWC approaches 100% for natural gas fuelled engines. The addition of TWCs to natural gas fuelled CHP engines approaches the environmental ideal of high overall efficiency, low emissions of NO,, negligible emissions of SO, and particulates, and the minimum emission of greenhouse gases per unit of generated energy.

ACKNOWLEDGEMENTS

The authors are grateful to Dr R. G. S. Banks for analytical support. The permission of British Gas plc to publish this paper is acknowledged; however, all opinions expressed are solely those of the authors.

REFERENCES

I 2 2a

2b

2c 2d 3 4 5

Lenner, M. Atmospheric Environment 1987, 21, 37 MacKenzie, D. New Scientist 1989, 122(1669), 36 Zemansky, M. W. and Dittman, R. H. ‘Heat and Thermo- dvnamics’. McGraw-Hill. 1981. DD. 142-144 and 227-229 dlarborg, P., Miller, J. ‘A. and ‘Kee, R. J. Comhusl. Flame 1986, 65, 177 and references therein Bowman, C. T. Prog. Energy Combust. Sci. 1975, 1, 33 Fenimore, C. P. 13th Symp. Combus. 1971, 373 Klimstra, J. SAE Technical Paper Series 872165 Gould, R. and Gribbin, 3. New Scienrisf 1989, 122(1665), 34 Prigent, M. Presented at European Workshop on N,O Emissions, Rueil-Malmaison, France, June 1988 Tidona, R. J., Nizami, A. A. and Ceransky, N. P. J. Air Poll. Conf. Assocn. 1988, 38, 806 Montgomery, T. A., Samuelson, G. S. and Muzio, L. J. J. Air Poll. Cont. Assocn. 1989, 39, 721 Tennent, R. M. (Ed.) ‘Science Data Book’, Oliver & Boyd, Edinburgh, UK, 1971, p. 78 Oh, S. H., Fisher, G. B., Carpenter, J. E. and Goodman, D. W. J. Cat. 1986, 100,360 Muzio, L. J., Teague, M. E., Kramhch, J. C., Cole, J. A., McCarthy, J. M. and Lyon, R. K. J. Air Poll. Conl. Assocn. 1989, 39, 287

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