Combustion of simulated biogas in a dual-fuel diesel engine

Combustion of simulated biogas in a dual-fuel diesel engine

PII: Energy Convers. Mgmt Vol. 39, No. 16±18, pp. 2001±2009, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0196-890...

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PII:

Energy Convers. Mgmt Vol. 39, No. 16±18, pp. 2001±2009, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0196-8904(98)00071-5 0196-8904/98 $19.00 + 0.00

COMBUSTION OF SIMULATED BIOGAS IN A DUAL-FUEL DIESEL ENGINE A. HENHAM* and M. K. MAKKAR$ School of Mechanical and Materials Engineering, University of Surrey, Guildford, GU2 5XH, U.K. AbstractÐTechnology related to biogas has been steadily developed over the last 50 years from small individually designed units to larger production plants. The development, however, has largely taken place on the side of biogas production and anaerobic waste treatment. Utilization of the gas produced by these methods has only recently been the subject of more scienti®c evaluation. The transformation of energy through biogas into the thermodynamically higher valued mechanical energy successfully and economically is now the most important research area in this ®eld. Of the engine work already published, most concerns spark-ignited engines. The authors' research work concerns the use of biogas in dual-fuel diesel engines. It examines engine performance using simulated biogas of varying quality representing the range of methane:carbon dioxide composition which may be encountered in gas from di€erent sources. The total programme includes the e€ects of biogas quality and of the proportion of energy from pilot fuel injection over a range of speeds and loads, investigations into the performance parameters over a range of compositions of gaseous mixture. A twocylinder, indirect-injection diesel engine of stationary type is being used as the ®rst experimental test bed in this work and the variation of quality is provided by mixing natural gas and carbon dioxide. A data acquisition system for in-cylinder pressure and crank angle is being used successfully and some emissions measurements are also available, particularly for CO and O2. One of the authors is from India where there is thought to be considerable potential for exploiting the gaseous products from resources such as biogas, land®ll and sewage gas through small stationary dual-fuel engines for irrigation and CHP applications. The nature of combustion process in the dualfuel engine is examined by the authors through pressure-crank angle data and studies of characteristics a€ecting engine eciency. # 1998 Elsevier Science Ltd. All rights reserved Biogas

Dual-fuel engine

Alternative fuels

INTRODUCTION

The gaseous fuels are getting more positive response from researchers and end-users compared with the past because of current unfolding developments. The ®rst development of importance is certainly the issue of the 1990 sÐthe environment. Gas is clearly the fossil fuel of least environmental impact. When burnt, it produces virtually no SOx and relatively little NOx, the main constituents of acid rain, and substantially less CO2, a key culprit in the greenhouse debate, than most oil products and coal. The second unfolding development is driven by technology. There has been a steady increase in the use of alternative transportation fuels. Our main emphasis is on the gaseous fuels. Use of natural gas for power generation in combined cycle plant has led thermal eciency to 52% while it is only 40% from state-of-the-art coal or oil ®red power plants which also require desulphurization. Other gases like biogas, land®ll gas and sewage gas have also attracted the researchers worldwide to realise and tap their energy potential to the optimum use. The analysis of the various gaseous fuels from the various sources like natural gas, biogas, land®ll and sewage gas reveals that the main constituent contributing to the heating value of the fuel is methane. Thus methane number can be used to classify the various gaseous fuels in similar ways to octane number and cetane number being used for petrol and diesel respectively. The focus of the present research is not only the use of biogas in internal combustion engines already explored very well by so many researchers [3, 4, 6±10] but to explore the e€ects of varying the quality of gaseous fuel in terms of the methane number of the fuel by mixing natural gas and carbon dioxide in di€erent proportions while using gasoil as pilot fuel. It will be very *To whom all correspondence should be addressed. $ Currently at Thapar Corporate R&D Centre, India. 2001

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signi®cant to obtain the relationship of methane number versus eciency of dual fuel engine and to compare the same with the diesel engine eciency.

MODIFICATION IN INTERNAL COMBUSTION ENGINES WORKING ON GASEOUS FUELS

The modi®cation of a spark ignition engine is comparatively easy as the engine is designed to operate on air/fuel mixture with spark ignition. The basic modi®cation is the provision of a gas±air mixer instead of the carburettor. The engine control is performed by the variation of mixture supply, i.e. throttle valve position as has been the case with petrol fuel. Spark ignition engines converted to natural gas show a power decrease of 15±20% attributed to a decrease in volumetric eciency because of the gaseous fuel and the lower ¯ame speed of air±gas mixture compared with air±gasoline mixtures. This power loss can be decreased to some extent by utilising the higher compression ratio possible with gas and advancement in spark timing. In stationary applications this loss of power is less important as they are mainly run at full load. In dual-fuel diesel engines, the normal diesel fuel injection system still supplies a certain amount of diesel fuel. The engine however induces and compresses a mixture of air and gaseous fuel which has been prepared in the external mixing device. The mixture is then ignited by energy from the combustion of the diesel fuel sprayed in. The diesel fuel spray is termed as pilot fuel. The amount of diesel fuel needed for sucient ignition is between 10±20% of the amount needed for operation on diesel alone at normal working loads. It di€ers with the point of operation and engine design parameters. Operation of the engine at partial load requires a reduction of the fuel gas supply by means of a gas control valve. A simultaneous reduction of the air supply would, however, decrease the quantity induced hence the compression pressure and the mean e€ective pressure. This would lead to a drop in power and eciency. With drastic reduction the compression conditions might even become too weak to e€ect self-ignition. Dual fuel engines should, therefore, not be throttled/controlled on the air side. Biogas as a fuel for vehicles has been an issue since the 1950 s. While in Europe the use in tractors seems to be the issue [1, 2] in Brazil the aim is to substitute petrol and diesel fuel in the automotive sector using puri®ed and compressed biogas or natural gas [11]. Biogas originates from bacteria in the process of biodegradation of organic material under anaerobic conditions and can also be produced by partial combustion of biomass in a gasi®er. A typical dry-gas composition [6] may be 18±20% CO, 8±10% CO2, 18±20% H2, 2±3% CH4 and a balance of N2. The widely variable composition of the gas from the gasi®er makes this fuel better suited to diesel engines operating in a dual fuel mode. Mukunda et al. [6] have discussed the complete gasi®er/diesel engine system in some detail. Stone et al. [8] have analysed biogas combustion (typical composition is 35% CO2 with 65% CH4) in spark-ignition engines by means of experimental data and a computer simulation. ``Ideally, there is a need for optimum variation in the liquid fuel quantity used any time in relation to the gaseous fuel supply so as to provide for any speci®c engine the best performance over the whole load range desired'' [3]. Usually, the main aim, for both emissions and economic reasons, is to minimize the use of the diesel fuel and maximize its replacement by the cheaper gaseous fuel throughout the whole load range. The dual-fuel engine can operate e€ectively on a wide range of di€erent gaseous fuels while maintaining the capacity for operation as a conventional diesel engine. Normally, the change over from dual fuel to diesel operation and vice versa, can be made automatically even under load.

EXPERIMENTAL SET-UP

The test engine for the present research work is a two-cylinder, four-stroke, water-cooled, indirect injection Lister Petter LPWS2 diesel engine. The set-up for experimental work including gas supply line with pressure cut-o€ and safety devices and other instrumentation used is shown in Fig. 1.

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Fig. 1. Set-up for experimental work and instrumentation.

Design for mixing device The mixing device was designed according to recommendations of von Mitzla€ [5]. It is a Tjoint with the gas pipe protruding into the device as shown in Fig. 2. The gas pipe is cut oblique with the opening facing the engine inlet. The protruding section increases the active pressure drop for the gas to ¯ow into the mixing device. The pressure drop increases further with increase in engine speed and thus sucks more gas also. The design calculations to evaluate diameter of the pipe for gas inlet are based on the parametersÐrated power, cubic capacity, rated speed, volumetric eciency, manifold diameter, diesel substitution, gas calori®c value and velocity of gas. To improve mixing further a turbulence grid shown in Fig. 3 has been introduced in the above mixing device.

Fig. 2. Gas mixing device.

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Fig. 3. Turbulence grid.

Fig. 4. IDI dual-fuel eciency variation using gas mixture at 2000 rev/min and 40 Nm.

Fig. 5. IDI dual-fuel exhaust temperature variation using gas mixture at 2000 rev/min and 40 Nm.

HENHAM and MAKKAR: COMBUSTION OF SIMULATED BIOGAS

Fig. 6. IDI dual-fuel CO variation using gas mixture at 2000 rev/min and 40 Nm.

Fig. 7. IDI dual-fuel eciency variation using gas mixture at 2800 rev/min and 40 Nm.

Fig. 8. IDI dual-fuel exhaust temperature variation using gas mixture at 2800 rev/min and 40 Nm.

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Fig. 9. IDI dual-fuel CO variation using gas mixture at 2800 rev/min and 40 Nm.

Fig. 10. P-y diagram for gasoil only @ 2000 rev/min, 40 Nm.

Fig. 11. P-y diagram for gasoil and 60% NG substitution @ 2000 rev/min, 40 Nm.

HENHAM and MAKKAR: COMBUSTION OF SIMULATED BIOGAS

Fig. 12. P-y diagram for gasoil and NG:CO2 (1:1) @ 2000 rev/min, 40 Nm.

Fig. 13. P-y diagram for gasoil only @ 2800 rev/min, 40 Nm.

Fig. 14. P-y diagram for gasoil and 60% NG substitution @ 2800 rev/min, 40 Nm.

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Fig. 15. P-y diagram for gasoil and NG:CO2 (1:1) @ 2800 rev/min, 40 Nm.

RESULTS AND DISCUSSIONS

Tests have been conducted on the dual-fuel diesel engine at various proportions of gas mixture comprising of natural gas (NG) and carbon dioxide. Firstly gasoil substitution by NG from the British mains (94% methane, 34.8 MJ/m3) was at four constant levels of substitution 22%, 37%, 45% and 58%. Then taking each constant level of NG as 100%, it has been mixed with CO2 to vary the composition of gas mixture. Each symbol represents a result at a particular proportion of energy from gaseous fuel. Test results in Fig. 4±6 are at engine speed 2000 rev/min and torque 40 Nm using NG:CO2 mixture for a range of 100:0 to 40:60 at four constant NG substitution levels. The overall eciency has been calculated on the basis of power obtained by the rate of energy input from gasoil and gas mixture at various proportions. It falls with NG substitution at all constant levels. On mixing NG with CO2 eciency is not much a€ected upto 37% NG substitution. With higher NG substitution eciency decreases with increasing CO2 in gas mixture. At 58% NG substitution level, eciency decreases from 28.2% to 26.2% with increasing CO2 in gas mixture. With higher gas substitution a greater proportion of air is replaced by gas so volumetric eciency is lowered resulting in less power. Figure 5 indicates that exhaust temperature is a€ected more by NG substitution up to 45%. At 58% NG substitution, exhaust temperature increases with increasing CO2 in gas mixture, from 3828C to 4028C. Figure 6 indicates that CO is a€ected mainly by NG substitution and not so much by the proportion of CO2 in the gas mixture. The increase in CO as compared to that with gasoil only where it was only 0.04% is caused by lower e€ective air fuel ratio as gas mixture replaces more air. Test results in Fig. 7±9 are at engine speed 2800 rev/min and torque 40 Nm using NG: CO2 mixture for a range of 100:0 to 30:70 at ®ve constant NG substitution levels. Figure 7 indicates that, at 2800 rev/min, overall eciency decreases with increase in CO2 in gas mixture at all substitution levels. Figures 8 and 9 indicate that exhaust temperature and CO follow the same patterns as at 2000 rev/min except at 65% NG substitution. At this condition the combustion is less controlled and knock was noticed during the test run. Figures 10±12 show the in-cylinder pressure characteristics of the test engine at gasoil only, gasoil and 58% NG substitution and gasoil and gas mixture (NG:CO2::1:1) respectively at 2000 rev/min and 40 Nm. Peak pressure rises from 70 bar to 83 bar at 58% NG substitution and falls to 77 bar for gas mixture of NG:CO2 (1:1). Sharper peaks may be observed in Figs 11 and 12 compared to Fig. 10 and that is thought to be the result of more fuel being available at the initiation of combustion. Figure 13±15 show the in-cylinder pressure characteristics of the test engine at gasoil only, gasoil and 60% NG substitution and gasoil and gas mixture (NG:CO2::1:1) respectively at 2800 rev/min and 40 Nm. Peak pressure rises from 57 bar to 70 bar at 60% NG substitution and falls to 67 bar for gas mixture of NG:CO2 (1:1). The di€erences from the shapes of the diagrams

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at 2000 rev/min derive from the injections required for the larger crank angle movement during increased power and speed. CONCLUSIONS

The extensive range of tests which have been conducted in varying fuel quality in a dual-fuel IDI engine have shown the following characteristics: . 60% gasoil substitution is possible by gas mixture without knock. . Overall eciency falls with gas mixture substitution and adding CO2 a€ects this more at higher speed. . Exhaust temperature is a€ected more by NG substitution than by CO2 addition except at maximum NG substitution. . CO is a€ected mainly by NG substitution and less by gas quality. . There is a more rapid pressure rise on combustion with dual-fuel operation. The authors are currently exploring the e€ect on combustion of using a direct-injection version of the same engine. AcknowledgementsÐThe authors are grateful for assistance in various ways from Lister Petter Diesels, British Gas and BOC. The British Council is supporting Mr Makkar's research studies under ``Nehru Centenary British Fellowship'' scheme.

REFERENCES 1. Buttner, S. and Mauser, K., Traktor mit Biogasantrieb-Umrustung and erste. Eiensatzerfahrungen. Landtechnik Nr. 6, KTBL, Darmstadt, FRG (in German), 1982. 2. Fankhauser, J. and Moser, A., Studie uÈber die Eignung von Biogas als Treibsto€ fur Landwirtschaftstraktoren. FAT publication no. 18. Tanikon, Switzerland (in German), 1983. 3. Karim, G. A., Automotive Engine Alternatives. The Dual Fuel Engine, ed. Robert L. Evans. Plenum Press, New York and London. 4. Kulkarni, M. K., Kirlosker dual fuel biogas engines, Commonwealth Regional (Asia/Paci®c) Rural Technology Programme, Bombay, India, 1980. 5. Von Mitzla€, K. Engines for biogas. A publication of Deutsches Zentrum fur Entwicklungstecknologien, GATE. 6. Mukanda, H. S., Dasappa, S. and Shrinivasa, U., Open-top Wood Gasi®ers, Renewable Energy, Earthscan Publications, London, 1993. 7. Sasse, L., Biogas Plants, GATE/Vieweg, Braunschweig, FRG, 1984. 8. Stone, C. R., Gould, J. and Ladommatos, N., J. Inst. En., 1993, 66, 180. 9. Werner, U. et al., Praktischer Leitfaden fur Biogasanlagen in der Tierproduktion. Oekotop/GATE. Eschborn, FRG (in German), 1986. 10. Zexi, C., Application of biogas on farm internal combustion engine, Provincial Agricultural Machinery Research Institute of Sichuan, P.R. of China, 1982. 11. Encontro de Biogas Automotive para Empresa Rural, 1-Londrina-PR, Embrater, Brazilia, Brazil (in Portuguese), 1983/84.