Datta, Souvik

Greencorn, Michael J.

Hargreaves, Justin S.J.

Jackson, S. David

Paul, Manosh

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          Greencorn, M. J., Jackson, S. D., Hargreaves, J. S.J., Datta, S. and Paul, M. (2020) Modelling the performance of a syngas fuelled engine: effect of excess air and CO2 as combustion diluents. Low-Carbon Combustion Joint Meeting of the French and British Sections of The Combustion Institute, 5-6 Nov 2020.   There may be differences between this version and the published version. You are advised to consult the publisher’s version if you wish to cite from it.     http://eprints.gla.ac.uk/225783/            Deposited on 29 October 2020  Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk                Modelling the performance of a syngas fueled engine: Effect of excess air and CO2 as combustion diluents.  Low-Carbon Combustion: Joint Meeting of the British and French Sections of the Combustion Institute, 5-7 November 2020, Lille, France.  Bio-syngas, derived from the pyrolysis and gasification of a biomass feedstock, can be used as an alternative fuel in reciprocating engines. This mixture of CO, H2, CH4, and CO2 is less calorific than traditional fuels and has different in-cylinder combustion properties, ultimately resulting in a power derating for syngas engines when compared to traditional engines of a similar size. Nevertheless, syngas remains an attractive renewable, low-carbon fuel suitable for use in spark-ignited (SI) internal combustion engines. The current study investigates syngas engine performance across varying air/fuel ratios and the effect of substituting CO2 for excess air. In the present work, a representative time-dependent, thermodynamic model of a four-stroke internal combustion engine is derived (eqs. 1 – 3). Fuel combustion is sub-modelled using a Weibe function tuned specifically for the combustion characteristics of syngas mixtures of predominantly H2, CO, and CO2 in air (eq. 4). Instantaneous heat lost to the cylinder wall is calculated using a convective heat transfer model with the heat transfer coefficient determined using a Nusslet-Reynolds correlation (eq. 5). 𝑑𝑃𝑑𝜃=𝛾 − 1𝑉⋅ (Δ𝐻𝑐 ⋅𝑑𝑋𝑏𝑑𝜃−𝑑𝑄𝐿𝑑𝜃) −𝛾 ⋅ 𝑃𝑉⋅𝑑𝑉𝑑𝜃+𝛾 − 1𝑉(𝛾0 𝑇0 𝑅𝛾0 − 1) ⋅𝑑𝑚𝑑𝜃   (1) 𝑉(𝜃) = 𝑉𝑐 +𝑉𝑐  (𝑟𝑐 − 1)2(1 − cos(𝜃) + 𝜖 ⋅sin2(𝜃)2) (2) 𝑑𝑉𝑑𝜃=𝑉𝑐  (𝑟𝑐 − 1)2sin(𝜃) ⋅ (1 + 𝜖 cos(𝜃)) (3) 𝑋𝑏(𝜃) = 1 − exp−2.23(𝜃−𝜃𝑠𝜃𝑑)1.71 (4) ℎ𝑐 = 𝑎𝐵𝑏−1𝑘(𝜌 ?̅?𝑝 𝜇)𝑏 (5)  When validated against experimental data (Shivapuji & Dasappa, 2013), this model can predict brake power output and in-cylinder pressure profiles, as highlighted in Table 1. Additionally, the model predicts the overall energy distribution between the brake power, friction losses, jacket cooling load, and exhaust sensible heat as shown in Table 2 (Shivapuji, 2015). Table 1: Experimental and modelled values for brake power output at different engine loads (Shivapuji & Dasappa, 2013) Measured brake power (experiment) 27.32 kW 16.41 kW 5.23 kW Calculated brake power (simulation) 27.25 kW 16.83 kW 5.30 kW Relative error 0.26% 2.56% 1.34% Table 2: Experimental and modelled energy distribution for an engine with 123.5 kW input energy rate (Shivapuji, 2015)  Brake power Cooling load Exhaust sensible heat Friction losses Measured output (experiment) 22.1% 31.9% 26.2% 14.8% Calculated output (simulation) 20.0% 32.5% 30.8% 15.6%  Engine simulation was set up by maintaining a constant mass flow of the syngas mixture while varying the mass flow air from sub-stoichiometric conditions to an excess air-fuel ratio of λ=2.15, representing the lean limit for syngas combustion (Dasappa, Sridhar & Paul, 2011). Intake conditions were set to constant temperature and pressure conditions of 298 K and 100 kPa, respectively. To simulate the effect of using CO2 as a substitute diluent instead of excess air, a mass flow stream of CO2 was introduced into the intake mixture while a stoichiometric airflow was maintained. The mass flow rate of CO2 was increased for successive iterations of the model. For comparison to excess air cases, an equivalent λ parameter is calculated for the CO2 cases such that the mass flow of air remains at the stoichiometric flow rate while the CO2 mass flow substitutes the equivalent mass flow of excess air. Equation (6) illustrates the calculation. 𝜆𝑒𝑞 = (1 +?̇?𝐶𝑂2?̇?𝑎𝑖𝑟,𝑠𝑡) (6) Under stoichiometric combustion conditions, the peak cylinder temperature is calculated to be 1941.4 K. For engine operation at excess air of λ=2.0, peak temperature is limited to 1519.1 K. This falls to a peak temperature of 1470.3 K for an equivalent CO2 dilution of λeq=2.0.  By substituting CO2 for excess air as the engine diluent gas, the indicated mean effective pressure at high λ ratios remains slightly higher, indicating more available work per engine cycle. For λ(eq)=1.50, this translates to a 1% increase in IMEP with a trend that rises to 2% for λ(eq)=2.15.  Additionally, lower cylinder cooling loads are required and exhaust gas temperatures remain higher under the CO2 diluent regime. For example, at λ(eq)=2.0 the cooling load reduces from 37.48 kW in excess air to 36.56 kW in CO2 while exhaust temperatures are 629.7 K and 688.2 K, respectively. This is attributable to the increased heat capacity of the exhaust gases since they have a higher CO2:N2 ratio. Such an effect is beneficial for potential down stream processes like removal of pollutants or CHP applications.      References: Dasappa, S., Sridhar, G., and Paul, P.J. (2011) “Adaptation of small capacity natural gas engine for producer gas operation”, Proc. IMechE Part C: J. Mechanical Engineering Science (226), pp. 1568-1578 Shivapuji, A. (2015) In-cylinder experimental and modeling studies on producer gas fuelled operation of spark ignited gas engines, Doctoral thesis, Indian Institute of Science Bangalore, p. 95 Shivapuji, A.M., and Dasappa, S. (2013) “Experiments and zero D modeling studies using specific Wiebe coefficients for producer gas as fuel in spark-ignited engines”, Proc IMechE Part C: J Mechanical Engineering Science 227(3), pp. 504-519 

Modelling the performance of a syngas fuelled engine: effect of excess air and CO2 as combustion diluents

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Modelling the performance of a syngas fuelled engine: effect of excess air and CO2 as combustion diluents

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