Preventive knock protection technique for stationary SI engines fuelled by natural gas

In a combined heat and power (CHP) plant, spark ignition engines must operate at their maximum power to reduce the pay back time. Because of environmental and economic concerns, engines are set with high compression ratios. Consequently, optimal operating conditions are generally very close to those of knock occurrence and heavy knock can severely damage the engine piston.There are two main protection techniques: the curative one commonly used by engine manufacturers and well documented in the literature and the preventive one based on a knock prediction according to the quality of the supplied gas. The indicator used to describe gas quality is the methane number (MN). The methane number requirement (MNR) of the engine is defined, for an engine set (spark advance, air-fuel ratio, and load), as the minimum value of MN above which knock free operation is ensured. To prevent knock occurrence, it is necessary to adapt the engine tuning according to variable gas composition. The objective of the present work is to validate the concept of knock preventive protection. First, a prediction of MNR according to engine settings (ES) is computed through a combustion simulator composed of a thermodynamic 2-zone model. Predicted MNR are compared to experimental results performed on a single-cylinder SI gas engine and show good agreement with numerical results (uncertainty below 1 point). Then, the combustion simulator is used to generate a protection mapping. At last, the knock preventive protection was successfully tested.     

Methane number testing of alternative gaseous fuels

Alternative gaseous fuels, like syn-gas and bio-gas, are attractive fuels for internal combustion engines due to energy and environmental concerns. Although the worldwide use of alternative gaseous fuels has increased, the knock properties of these fuels are not well understood. The methane number (MN) knock rating technique was selected based on its range and sensitivity. Eight alternative gaseous fuel compositions were simulated with a gas blending system and tested for MN in a Cooperative Fuel Research (CFR) F-2 engine. The alternative gaseous fuels ranged from 24 to 140 MN (natural gas typical range 75-95).     

Efficient Generation of Electricity from Methane using High Temperature Fuel Cells – Status,Challenges and Prospects

High temperature fuel cells (HTFCs), comprising solid oxide fuel cells and molten carbonate fuel cells, present efficient means for generating electricity from methane and natural gas. The high quality heat generated by HTFCs allows operation in the combined heat and power mode (CHP) to further enhance efficiency. The overall fuel-to-electricity conversion efficiency of an HTFC system operating in CHP mode can approach up to 80 %. Despite the high operating efficiency of HTFCs, high capital costs and durability issues have hindered their widespread commercialization. This article provides an overview of the operating principles, technical challenges, commercialization status of HTFCs, and outlines the strategies being adopted to lower capital costs and increase durability.     

Mass transfer behavior of methane in porous carbon materials

The low mass transfer rate in porous materials hinders the use of adsorbed natural gas as vehicle fuel. Fundamentally, the mass transfer rate depends on the structures of the adsorbents and the operating conditions. Therefore, in this study, the effects of adsorbent (activated carbons) structure and operating conditions on the mass transfer rate of methane (main component of natural gas) were investigated quantitatively, providing a theoretical basis for the synthesis of efficient adsorbent materials. By performing Monte Carlo and molecular dynamics simulations and utilizing a nonequilibrium thermodynamic linearization transfer model, the mass transfer behavior of methane in porous carbon materials was quantitatively evaluated, specifically focusing on the material structure, operating conditions, and feasibility of using natural gas as vehicle fuel. The proposed linear nonequilibrium thermodynamic mass transfer model is applicable to interfacial gas species and provides a valuable tool for gas separation.     

Knock rating of gaseous fuels in a single cylinder spark ignition engine

This paper presents the determination of knock rating of gaseous fuels in a single cylinder engine. The first part of the work deals with an application of a standard method for the knock rating of gaseous fuels. The Service Methane Number (SMN) is compared with the standard Methane Number (MN) calculated from the standard AVL software METHANE (which corresponds to the MN measured on a Cooperative Fuel Research engine). Then, in the second part, the ‘mechanical’ resistance to knock of our engine is highlighted by means of the Methane Number Requirement (MNR). A single cylinder LISTER PETTER engine was modified to run as a spark ignition engine with a fixed compression ratio and an adjustable spark advance. Effects of engine settings on the MNR are deduced from experimental data and compared extensively with previous studies. Using the above, it is then possible to adapt the engine settings for optimal knock control and performances. The error on the SMN and MNR stands beneath ±2 MN units over the gases and engine settings considered.     

A study of natural gas/DME combustion in HCCI engines using CFD with detailed chemical kinetics

Combustion characteristics of natural gas and dimethyl ether (DME) mixture in a homogeneous charge compression ignition (HCCI) engine were studied numerically. Detailed chemical kinetics with 83 species and 360 reactions was used with an engine CFD code to simulate the combustion process. Operating conditions with different fuel compositions were simulated. Combustion, nitrogen oxides emissions and effects of fuel compositions on engine operating limits were well predicted by the present model. Chemical kinetics analysis indicated that ignition was achieved by DME oxidation which, in turn, induced combustion of natural gas. Low-temperature heat release is more pronounced as the amount of DME increases. Engine operations become unstable as the excess air ratio of natural gas is reduced near 2. The model also captures the HCCI features of low-combustion temperature and low-nitrogen oxides emissions for the alternative fuels used in this study.     

Combustion of methane injected into an air flow with high subsonic velocities by different methods

An important stage of the development of promising engine and propulsion systems is provision of an effective process of hydrocarbon fuel combustion. There are many publications with numerical and experimental data on combustion of various gaseous hydrocarbons under laboratory conditions, but there is a lack of data on effective combustion of hydrocarbons in short combustion chambers with a large number of injectors. Results of systematic experimental studies of natural gas (methane) combustion in a high-velocity subsonic air flow in an air-breathing model combustor with a rectangular cross section are presented in this paper.     

NOx reduction from a large bore natural gas engine via reformed natural gas prechamber fueling optimization

Lean combustion is a standard approach used to reduce NO_x emissions in large bore (35–56 cm) stationary natural gas engines. However, at lean operating points, combustion instabilities and misfires give rise to high total hydrocarbon (THC) and carbon monoxide (CO) emissions. To counteract this effect, precombustion chamber (PCC) technology is employed to allow engine operation at an overall lean equivalence ratio while mitigating the rise of THC and CO caused by combustion instability and misfires. A PCC is a small chamber, typically 1–2% of the clearance volume. A separate fuel line supplies gaseous fuel to the PCC and a standard spark plug ignites the slightly rich mixture (equivalence ratio 1.1–1.2) in the PCC. The ignited PCC mixture enters the main combustion chamber as a high energy flame jet, igniting the lean mixture in the main chamber. Typically, natural gas fuels both the main chamber and the PCC. In the current research, a mixture of reformed natural gas (syngas) and natural gas fuels the PCC. Syngas is a broad term that refers to a synthetic gaseous fuel. In this case, syngas specifically denotes a mixture of hydrogen, carbon monoxide, nitrogen, and methane generated in a natural gas reformer. Syngas has a faster flame speed and a wider equivalence ratio range of operation than methane. Fueling the PCC with Syngas reduces combustion instabilities and misfires. This extends the overall engine lean limit, enabling further NO_x reductions.Research results presented are aimed at quantifying the benefits of syngas PCC fueling. A model is developed to calculate the equivalence ratio in the PCC for different mixtures and flowrates of fuel. An electronic injection valve is used to supply the PCC with syngas. The delivery pressure, injection timing, and flow rate are varied to optimize PCC equivalence ratio. The experimental results show that supplying the PCC with 100% syngas improves combustion stability by 21% compared to natural gas PCC fueling. A comparison at equivalent combustion stability operating points between 100% syngas and natural gas shows an 87% reduction in NO_x emissions for 100% syngas PCC fueling compared to natural gas PCC fueling.     

Experimental study of a reverse flow catalytic converter for a dual fuel engine

Theperformance of a reverse flow catalytic converter for a methane/diesel dual fuel engine, a monolith honeycomb converter with palladium catalyst washcoat, was evaluated under steady and transient engine conditions. The reverse flow converter provided superior performance (that is, higher conversion of pollutants) for several steady engine operations, compared with unidirectional flow operation. For transient operation following a step change in engine operating conditions, reverse flow is better than unidirectional flow when the change in engine operation results in a reduction in exhaust gas temperature. For an increasing exhaust gas temperature, reverse flow decreased the rate of increase of reactor temperature. The reverse flow converter was tested using the transient Japanese 6‐Mode tests. Reverse flow operation gave higher conversions than unidirectional flow for this test, with a switch time of 5 s giving the best results.     

Spark-ignition engine performance with ‘Powergas’ fuel (mixture of CO/H2): A comparison with gasoline and natural gas

Results are presented of tests from a variable compression ratio Ricardo E6 single-cylinder spark-ignition (SI) engine operating on ‘Powergas’—a synthetic fuel consisting mainly of carbon monoxide and hydrogen. The tests cover a range of air/fuel ratios from rich to the lean operating limit at different speeds and two different compression ratios. Measured results are given for brake torque, brake specific fuel consumption and the concentrations of carbon monoxide (CO), oxides of nitrogen (NO_x) and total unburnt hydro-carbon (THC) emissions in the exhaust gases. Experimental results indicate that ‘Powergas’ produces about 20 and 30% lower engine power output than natural gas (NG) and gasoline fuelling respectively under similar operating conditions. For ‘Powergas’, concentrations of THC and CO in the exhaust were negligible, but carbon dioxide (CO₂) and NO_x were found to be higher compared to other fuels. The engine simulation program ISIS has been used to simulate some of the exhaust emissions and the results show agreement with the experimental values and help explain the experimental results.     

Thermodynamics of Hydrogen Generation from Methane for Domestic Polymer Electrolyte Fuel Cell Systems

Low temperature fuel cells such as the Polymer Electrolyte Fuel Cell (PEFC) are preferably used for domestic applications because of their moderate operating conditions. Using the existing distribution system, natural gas is used as a source for a hydrogen rich gas to power this fuel cell type. The high requirements on the fuel gas quality as well as high conversion efficiencies for the small local gas processing units are critical aspects in the evaluation of decentralized fuel cell systems. In the present paper, three typical gas processing methods are evaluated for the supply of a hydrogen rich gas for PEFCs: steam reforming, partial oxidation, and autothermic conversion. All three processes are studied in detail by varying the relevant process parameters: temperature, pressure, steam to fuel ratio, and oxygen to fuel ratio. The results are graphically displayed in numerous nomograms. With the help of these graphs, regions of stable operation and the sensitivity to the operational parameters are discussed. For all three gas processing methods, the graphs generated display methane conversion, the hydrogen yield, and the yields of unwanted components, i.e., carbon monoxide and solid carbon. Although only steady‐state operating conditions were simulated, critical modes of operation, which might occur during start‐up or transient operation can easily be identified. For instance, operating conditions where soot is generated have to be avoided under all circumstances. All simulations were done with the Gibb's reactor model of a commercial simulation program. The Gibb's reactor model was found to be a suitable tool, since the simulated results compared well with reported literature data. According to the simulation results, the methane‐steam‐reforming process appears to be favorable for application to PEFCs. Methane conversion and hydrogen yields are highest for this process while the yield of CO is relatively low.     

Development of an autoignition submodel for natural gas engines

This paper describes a new autoignition submodel for engine modeling codes. This submodel does not require extensive computational resources and is easily portable to various computational environments. It also considers variation of natural gas composition due to propane addition. Computation results show that the knock occurrence crank angle can be predicted within 2° CA when the model is coupled to a zero-dimensional engine model, which was also developed for the present work. The results with the model incorporated into a multi-dimensional model (KIVA) are also promising. KIVA was able to predict if the engine was going to knock or not and also gave correct trends in the knock intensity.     

An experimental investigation of CNG as an alternative fuel for a retrofitted gasoline vehicle

This paper presents test results obtained from running a 1.5 L, 4-cylinder Proton Magma retrofitted spark ignition car engine with dynamometer. Performance, fuel consumption and exhaust emissions measurements were recorded under steady state operating conditions for gasoline and compressed natural gas (CNG). The engine was converted to computer integrated bi-fueling system from a gasoline engine and was operated separately either with gasoline or CNG using an electronically controlled solenoid actuated valve system. A PC based data acquisition and control system was used for controlling all the operation. A comparative analysis of the performance and emissions has been made for gasoline and CNG. Based on the experimental results, it is transparent that CNG shows low brake mean effective pressure (BMEP), brake specific fuel consumptions (BSFC), higher efficiency and lower emissions of CO, CO₂, HC but more NO_x compared to gasoline.     

An Industrial Scale Dehydration Process for Natural Gas Involving Membranes

An industrial scale dehydration process based on hollow fiber membranes for lowering the dew point of natural gas is described in this paper. A pilot test with the feed flux scale of 12×10⁴ Nm³/d was carried out. Dew points of –8 °C∼–13 °C at a gas transport pressure in the pipeline of 4.6M Pa and methane recovery of more than 98% were attained. The water vapor content of the product gas could be maintained around 0.01 vol% during a continuous run of about 700 hours. The effects of feed flux and operation pressure on methane recovery and water vapor content were also investigated. Additionally, some auxiliary technologies, such as a full‐time engine using natural gas as fuel and the utilization of vent gas in the process, are also discussed. A small amount of the vent gas from the system was used as a fuel for an engine to drive vacuum pumps, and the heat expelled from the engine was used to warm up the natural gas feed. The whole system can be operated in a self‐sustainable manner from an energy point of view, and has a relatively high efficiency in the utilization of natural gas.     

Solubility of Natural Gas in Diesel Fuel

The solubility of commercially available natural gas in commercially available diesel fuel at room temperature and defined pressure is investigated experimentally. The gas phase is considered to be pure methane. The use of Henry's law to model the solubility is discussed. Solubility is given in terms of the mole fraction and the volumetric mass concentration of dissolved gas and the corresponding Henry's coefficients. The solubility is compared to that of pure methane in pure hexadecane, which is similar to diesel fuel with respect to the mean carbon number.     

Low-temperature autoignition of binary mixtures of methane with C<Subscript>3</Subscript>–C<Subscript>5</Subscript> alkanes

The influence of C₃–C₅ alkanes on the ignition of their binary mixtures with methane in air at a temperature of 523–1000 K and a pressure of 1 atm is studied. It is shown that the presence of only 1% C₃–C₅ alkanes considerably reduces the ignition delay of methane. At a concentration of 10–20%, the ignition delay practically corresponds to the autoignition delay of the added alkane. The effect of additives of heavy alkanes becomes less noticeable with increasing initial temperature. These results can be used to estimate the permissible content of C₅₊ heavy species in gas turbine engine fuel at which their influence on the fuel knock resistance is sufficiently low. It is only 0.5%.     

Effect of engine parameters and gaseous fuel type on the cyclic variability of dual fuel engines

This paper presents an analysis of the cycle-to-cycle combustion variation as reflected in the combustion pressure data of a single cylinder, naturally aspirated, four stroke, Ricardo E6 engine converted to run as dual fuel engine on diesel and gaseous fuel of LPG or methane. A measuring set-up consisting of a piezo-electric pressure transducer with charge amplifier and fast data acquisition card installed on an IBM microcomputer was used to gather the data of up to 1200 consecutive combustion cycles of the cylinder under various combination of engine operating and design parameters. These parameters included type of gaseous fuel, engine load, compression ratio, pilot fuel injection timing, pilot fuel mass, and engine speed. The data for each operating conditions were analyzed for the maximum pressure, the maximum rate of pressure rise—representing the combustion noise, and indicated mean effective pressure. The cycle-to-cycle variation is expressed as the mean value, standard deviation, and coefficient of variation of these three parameters. It was found that the type of gaseous fuel and engine operating and design parameters affected the combustion noise and its cyclic variation and these effects have been presented.     

Fuel and emissions properties of Stirling engine operated with wood powder

From viewpoints of the environment and fuel cost reduction, small-scale biomass combined heat and power (CHP) plants are in demand, especially wood-waste fueled system, which are simple to operate and maintenance-free with high thermal efficiency similar to oil fired units. These are requested by wood and other industries located in mountainous region. To meet these requirements, a Stirling engine CHP system combined with simplified biomass combustion process with pulverized wood powder was developed.In an R&D project started in 2004 considering wood powder properties as a fuel, combustion performance and emissions in combustion flue gas were tested using combustion test apparatus with commercial size units. The wood powder combustion system was modified and optimized during the combustion test results, and the design of the demonstration plant combined with 55 kW_e Stirling engine power unit was considered. The demonstration plant was finally completed in March of 2006, and test operation has been progressed for the future commercial CHP system.In the wood powder combustion test, wood powder of less than 500 μm is mainly used, and a combustion chamber length of 3 m is applied. In these conditions, the air ratio can be reduced to 1.1 without increasing CO emission of less than 10 ppm and combustion efficiency of 99.9%. In the same conditions, NO_x emission is estimated to be less than 120 ppm (6% O₂ basis). Wood powder was confirmed to have excellent properties as a fuel for Stirling engine CHP system. This paper summarizes the wood powder combustion test, and presents the evaluation of the burner design parameters for the biomass Stirling engine system.     

Implications of fuel selection for an SI engine: Results from the first and second laws of thermodynamics

The current work examines the detailed thermodynamics of the use of eight (8) fuels by an automotive, spark-ignition engine using a thermodynamic engine cycle simulation. The fuels examined were methane, propane, hexane, isooctane, methanol, ethanol, carbon monoxide, and hydrogen. Both overall engine performance parameters and detailed instantaneous quantities are determined for each of the fuels. Results include thermal efficiencies, heat transfer, and exhaust gas temperatures as functions of engine speed and load. In general, the overall engine results were similar for the various fuels. The second law results showed that, for the same operating conditions, the destruction of exergy during the combustion process ranged between about 8% (for carbon monoxide) and 21% (for isooctane) of the fuel exergy depending on the specific fuel. The differences of the exergy destruction during combustion appear to be related to the complexity of the fuel molecule and the presence (or lack) of oxygen atoms in the fuel molecule.     

Numerical simulation of the two-dimensional flow in high pressure catalytic combustor for gas turbine

The objective of this paper is modeling the mechanism of high pressure and high temperature catalytic oxidation of natural gas, or methane. The model is two-dimensional steady-state, and includes axial and radial convection and diffusion of mass, momentum and energy, as well as homogeneous (gas phase) and heterogeneous (gas surface) single step irreversible chemical reactions within a catalyst channel. Experimental investigations were also made of natural gas, or methane combustion in the presence of Mn-substituted hexaaluminate catalysts. Axial profiles of catalyst wall temperature, and gas temperature and gas composition for a range of gas turbine combustor operating conditions have been obtained for comparison with and development of a computer model of catalytic combustion. Numerical calculation results for atmospheric pressure agree well with experimental data. The calculations have been extended for high pressure (10 atm) operating conditions of gas turbine.