Thermodynamic simulation model for predicting the performance of spark ignition engines using biogas as fuel
Introduction
According to the International Energy Agency, the growth in world economy will cause a 30% increase in global energy demand, between 2013 and 2040 [1]. Although renewables are expected to have a greater share in the energy mix, the dominant source of energy will continue to be fossil fuels, which currently account for two thirds of total greenhouse gas (GHG) and 80% of CO2 emissions [2]. The ratification of the 2016 Paris agreement extended GHG emission mitigation obligations to all countries [3]. The commitment to meet these obligations and the growing concerns over energy security have renewed the interest in renewable and domestic energy resources that can be alternative or supplementary sources and reduce these emissions. Biogas, in particular, is a favorable alternative, when compared to fossil fuels, due to being renewable and able to be produced in different locations where biomass or organic residues are available.
Biogas is a cheap gaseous fuel composed primarily of methane (CH4) and carbon dioxide (CO2) and can be produced from the anaerobic decomposition of organic compounds [4]. It is considered CO2 neutral and the use of agriculture biogas inside the energy matrix contributes to reducing methane gas emissions, which is another GHG [5]. Biogas is combustible, due to the CH4 in its composition, and can therefore be used in engines developed for commercial purposes that operate with natural gas, which results in a growing interest in its use in rural areas. Luz et al. estimated that biogas can be used in Brazil for electricity and heat generation and can be scaled up for reasonable returns [6]. Porpatham et al. removed CO2 from biogas composition and used it in a spark ignition engine in lean regimes. The results showed that HC emissions were reduced while power output, thermal efficiency and combustion speed were increased for a given equivalence ratio with CO2 removal [7]. Removing carbon dioxide from its composition to reduce its volume also allows biogas to be used in transport [8]. As a vehicle fuel, biogas can reduce methane production resulting from manure decomposition as well as decrease fossil fuel emissions [9]. However, biogas has low calorific value due to the presence of non combustible substances, such as water and carbon dioxide, resulting in lower combustion efficiency. This issue is worsened by the great variability in biogas composition due to the many possible sources of the organic material used to produce it. Furthermore, the presence of hydrogen sulfide (H2S) is undesirable, due to its corrosive properties. Therefore, in order to be used in internal combustion engines, biogas must have less than 0.05% in volume of H2S [10]. Although the taxes levied on the fuel it replaces is the main determining factor in whether to use this gas, it must also compare reasonably well with the incumbent fuel with the least amount of modifications to the engine [8]. In internal combustion engines, in particular, the compared parameters are engine performance characteristics and emissions, specifically nitrous oxide (NOx), carbon monoxide (CO), carbon dioxide and hydrocarbons (HC).
Spark ignition (SI) engines can easily be adapted to operate with biogas. Compression ignition engines, on the other hand, require more sophisticated modifications as they need to operate in dual fuel mode [11]. Several authors have studied the performance and emissions of SI engines operating on this gas. Arroyo et al. noticed a decrease in specific CO and NOx emissions when using biogas compared to gasoline at boost pressure between 1 and 1.3 bar. Chandra et al. reported a decrease in brake power and an increase in specific gas consumption when using biogas compared to compressed natural gas (CNG). These trends were to be expected due to the relatively high CO2 content in biogas composition, which resulted in smaller heating value and thus lower combustion temperatures and combustion efficiency. Methane enrichment of biogas was shown to be an efficient solution in improving engine performance, however, NOx emissions tended to increase. Bedoya et al. reported the results obtained using a diesel engine modified to operate in homogeneous charge compression ignition (HCCI) mode and evaluated the effect of inlet charge temperature, boost pressures, and the equivalence ratio of the biogas air mixture on HCCI combustion parameters and emissions [12]. The results showed that HC and CO emissions were reduced for lower equivalence ratios and slightly higher inlet temperature and boost pressure. On the other hand, higher equivalence ratios resulted in smaller reductions in HC and CO emissions, while NOx emissions were higher as boost pressure and inlet temperature were increased. These NOx emissions were still under the limits for stationary operation and as a result, HCCI engines operating with biogas could be promising candidates for stationary operation. Subramanian et al. compared the mass emissions and fuel economy of a spark ignition engine operating with base compressed natural gas (CNG) and methane enriched biogas [13]. The vehicle’s emissions with both fuels met the BS IV Emission Norms, although enriched biogas had marginally higher HC, CO and NOx emissions than CNG. Enriched biogas, however, presented similar performance to CNG and could be used as fuel in spark ignition engine based vehicles. Crookes [14] reported that NOx emission of a spark ignition engine increased with enriched biogas as compared to raw biogas at relative air-fuel less than 1.15. CO emissions decreased with enriched biogas as compared to raw biogas. HC emissions increased with enriched biogas (CO2: 30%) as compared to raw biogas. Huang and Crookes [15] simulated biogas formed from different mixtures of domestic natural gas and carbon dioxide. The fraction of carbon dioxide in the simulated biogas was changed from 0 to about 40% by volume. They reported that the presence of carbon dioxide can improve NOx emissions, but since lower cylinder pressures result, engine power and thermal efficiency are reduced and the level of unburned HC is increased. Park et al. verified the effects of hydrogen (H2) addition on performance in a spark ignition engine operating with low calorific quality biogas. H2 addition improved the combustion process, extending the lean operating limit and reducing HC emissions, although NOx emissions increased [16]. Significant reduction in NOx emissions can be achieved by exhaust gas recirculation (EGR), which reduces peak temperature and could be used in spark ignition engines operating with natural gas [17]. However, power output decreases significantly when EGR is used for NOx control. Therefore, engine size must be increased to meet power requirements [18]. Although exhaust gas treatment could be used to achieve reasonable trade-off between performance and emissions, additional equipments and modifications are required. Emission control can also be achieved by varying some of the operating parameters, such as the operating equivalence ratio and spark ignition timing, or modifying the biogas composition, through hydrogen addition or methane enrichment.
Depending on the source of the fuel and its composition, experimental studies must be conducted to identify an engine’s ideal operating parameters and verify its viability [19]. However, experimentation usually involves many people, facilities and funds. Engine simulation presents a solution to this problem, minimizing unnecessary experiments and allowing for cost-effective, extensive parametric studies. Furthermore, simulation allows the optimization of engine parameters for a set of operating variables [20]. However, accurate prediction of exhaust gas emissions requires sophisticated models that can be computationally expensive. Furthermore, modifying these operating parameters results in moderate emission reductions and substantial performance losses [18]. Engine performance, on the other hand, is the main disadvantage of biogas when compared to fossil fuels. As such, a model that could predict engine performance trends within an operating range, while being able to be easily modified for different biogas compositions, could be a very useful tool in guiding experimentation.
With regard to engine simulation, one of the most important factors to consider is the combustion model to be used, which can be zero-dimensional, quasi-dimensional or multi-dimensional. Zero-dimensional models are the simplest way to observe the effects of variation of engine operating parameters on the pressure inside the cylinder and heat release rate. In the zero-dimensional formulation, the cylinder volume can be treated either as “single-zone”, “two-zone” or “multi-zone” [21]. Generally, single zone thermodynamic models are used for a quick analysis of engine performance. In this case, it is assumed that the cylinder charge is either an uniform or a homogeneous mixture with constant temperature and composition throughout the engine cycle. Such an approach is often used when simulating a spark ignition engine, due to the operating fluid being a homogeneous mixture of fuel and air. In modeling of the spark ignition engine and calculating its heat release, the specific heat ratio is the most important thermodynamic factor. Gatowski [22] developed a single zone heat release model based on the first law of thermodynamics which is widely used. Huffman [23] used the geometric relationship between the combustion chamber surface area and volume and the crank angle. This relationship was implemented in the differential formulation of the ideal gas law and the equations of the first law of thermodynamics to present the start and duration of heat release. These were used to analyze the effect of timing on spark and compression ignition engines by combining the ideal gas law with the heat release model. Yusaf [24] developed a single zone engine. Abu Nada [25] made a thermodynamic analysis of spark-ignition engines using a single zone model and considering the variation of specific heat called gas-mixture model [26]. Even though single zone models adopt simplifying assumptions in their formulations, they are still able to predict performance characteristics that are in agreement with experimental results.
The present paper reports a single zone, thermodynamic model to predict performance parameters of a SI engine fueled with biogas. This simulation was developed in order to reduce time and costs associated with the experimental testing needed to determine performance parameters in these engines. Furthermore, the simulation code is simple enough and can be adapted to model different compositions of biogas. Before the simulation, experimental performance and emission studies were done using biogas produced from raw material from the Alegria Station sewage treatment station. Equivalence ratio and spark ignition were varied during the experiments and its influence on parameters and emissions were then studied. These experimental measurements were then used as input in the simulation model and the results were compared to the experimental ones. Model validation was focused on the variation of power and specific consumption, while emissions were not accounted for, as comparison studies made by this research group showed that biogas NOx emissions were generally lower than those of natural gas. Furthermore, the experimental measurements verified the trends reported in literature and confirmed that the penalties to engine performance were more significant than the gains in emission reduction in the operating range tested. Thus, emission results obtained experimentally were used to determine the equivalence ratio () which was input in the model. The combustion process inside the combustion chamber was modeled based on empirical correlations for Wiebe equation’s parameters instead of fixed values reported in the literature. Finally, the model was validated with the experimental data and the results were discussed. The simulation model presented aimed to provide a tool that could predict engine performance for a set of operation conditions and biogas composition and could help guide experimentation to optimize performance, with minimal engine modifications or exhaust gas treatment requirements. As such, this tool is suitable for use with biogas supplies in developing regions, such as Brazil, for small-scale power generation in rural areas.
Section snippets
Experimental setup
Fig. 1 shows a schematic diagram of the experimental setup, which consisted of: an engine-generator unit, a gas analyzer, a load bank, a flow control valve/flowmeter admission system and a data acquisition system. The experimental apparatus was installed at the biogas production plant, part of the Alegria Station sewage treatment plant in Rio de Janeiro, Brazil. Thus, this engine-generator unit could be fueled directly on site, therefore avoiding the need to store and transport fuel.
However,
Experimental methodology and tests
The engine throttle opening was used to control the power developed. The engine-generator operated at a fixed nominal speed of 3600 RPM, employing different electric loads corresponding to the 4 different operating conditions (100%, 75%, 50% and 25%). Air-fuel ratio was varied from the lean burning limit to the rich side for those different loads (from = 0.94 (rich mixture) to = 1.06 (lean mixture)). This variation in air-fuel ratio was done by changing the fuel flow rate and keeping the air
Model description
According to Lanzafame et al. [28], [29], [30] zero-dimensional thermodynamic models can be more precise when they include temperature variation of specific heats ratio (k) and cylinder wall heat exchange [31]. The simplification assumptions were adopted according to the literature [32], [33], [34], [35], [36], [37], [38] and most of them are an intrinsic part of the zero-dimensional model [32], [39], [40]:
- 1.
The combustion chamber was considered an ideal cylinder;
- 2.
The air fuel mixture behaved as
Simulation input and calculation procedure
The simulation procedure used in this paper was done using Wolfram Mathematica 7 to solve the aforementioned differential equation system. Input to this model included engine geometric data, engine design information, fuel data, combustion parameters and engine operating parameters, and are presented in Table 3. The solution procedure was separated into three different steps, which were compression, combustion and expansion. Specific fuel consumption and indicated power were selected as output
Experimental results and model validation
As previously explained in Section 5, the experiments were carried out using the methodology presented in Section 3 and the emission results were used to determine the equivalence ratio, which was an input to the simulation model presented in Section 4. These experiments also provided performance results that were then validated with the proposed simulation model.
Conclusions
Biogas is a renewable fuel with great potential to be used in spark ignition internal combustion engines with minor modifications. Its main benefits consist in reducing GHG emissions substantially and being widely available. On the other hand, its low calorific value presents a drawback in terms of engine performance. Thus, in order for it to be a suitable replacement for natural gas or other fossil fuels in spark ignition IC engines, it is important to find the optimal operating parameters
Acknowledgement
We would like to present our thanks to Professor Silvio Carlos de Almeida for his contributions in the last revision of this paper.
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