Gas Engines Application and Installation Guide

1 Gas EnginesApplication andInstallationGuideG3600 G3300lFuelslFuel SystemsLEKQ7256 (Supersedes LEKQ2461)10-97G3600 G3300 FuelsFuel CharacteristicsHydrocarbonsStandard Condition of a GasHeat ValueMethane NumberAir Required for CombustionCommon FuelsNatural GasSour GasPropanePropane-Butane MixturesPropane-AirPropane Fuel Consumption CalculationsDigester GasSanitary Landfill GasManufactured GasesConstituents of Gas by Volume - PercentProducer GasIlluminating GasCoke-Oven GasBlast Furnace GasWood GasCleaningFuel Effects on engine PerformanceHeat Value of the Air-Fuel MixtureTurbocharged EnginesMethane Number Program CalculationsFuel ConsumptionDetonationMethane NumberCompression RatioIgnition TimingLoad Inlet Air TemperatureAir-Fuel RatioEmissionsVariations in Heating ValueFuel TemperatureRecommendationsFuel RequirementsHeating Value5 FuelsMost of the fuels used in internal combustionengines today, whether liquid or gaseous.

2 Arecomposed primarily of hydrocarbons(hydrogen and carbon); their source isgenerally petroleum. natural gas is the mostpopular and widely used of the petroleumgases. Digester gas (also a hydrocarbon) andsome manufactured gases (from coal), whichcontain hydrocarbons, are also used inengines with var ying degrees of gas is the most practical of themanufactured or by-product commercial fuel gas is a mixture ofgases, some combustible and some inert. Thedifferent mixtures have extremely widevariations in composition. Consequently, it isnecessar y to closely examine thecharacteristics and behavior of an CharacteristicsHydrocarbonsHydrocarbons are grouped into threeclassifications according to their molecularstructure. Paraffins - CnH2n+2 Napthenes - CnH2n Aromatics - CnH2n-6 Most of the important fuel gases used inengines today are of the Paraffin series. Thisincludes both natural gas and digester series starts with methane (CH4); eachsucceeding member of the series has onemore carbon (C) atom and the correspondingnumber of hydrogen (H) atoms, etc.

3 Thenormal Paraffin hydrocarbons are said tohave straight chainmolecular structures,having one bond between each atom. The firstfour of the Paraffin series would havestructures as follows:HH HH H HH H H HH-C-HH-C-C-HH-C-C-C-HH-C-C-C-C-HHH HH H HH H H HMethane EthanePropaneButaneCH4C2H6C3H8C4H10As the number of atoms increases, themolecular weight of the molecule increasesand the hydrocarbons are said to becomeheavier. Their physical characteristics changewith each change in molecular the first four of the Paraffin series areconsidered gases at standard conditions kPa ( psia) and C (60 F).Several of the others can be easily convertedto gas by applying a small amount of Condition of a GasIt is important to note that when standardconditions are referenced, it means kPa ( psia) and C (60 F).When a gaseous fuel flow is stated in SCF, itmeans standard cubic feet (or standard cubicmeters - SCM) and is referenced to a gas atstandard conditions.

4 In some places, Europefor example, gas is referenced to kPa( psia) and 0 C (32 F). When gases arereferenced to 0 C (32 F), the units are callednormal cubic meters (NM3) or normal cubicfeet (NF3).Heat ValueHeat value is defined as the amount of energy(heat) released during the combustion of afuel with the correct amount of oxygen (air).It is determined with a device called acalorimeter. A known quantity of fuel andoxygen are combined in a calorimeter andburned. Heat is generated and, the waterproduced (from the combustion of fuelscontaining Hydrogen; either CxHyor H2) iscondensed. The heat measured by thecalorimeter is called the high heat value of thefuel (also referred to as the gross heat value).It is important to understand the differencebetween high and low heat value since enginemanufacturers typically use low heat valuewhen discussing fuels and engine data. Adiscussion of heat value as it relates to theinternal combustion engine may help providea better understanding of the differencebetween high and low heat any hydrocarbon is used as fuel in aninternal combustion engine , one of theproducts of combustion is water.

5 The amountof water formed during combustion varies forthe different hydrocarbon fuels. This will beillustrated later. The water formed isconverted into steam by the combustion heatbefore leaving the engine , and carries with itthe quantity of heat used to convert the waterinto steam. This quantity of heat absorbed inchanging water, at a given temperature tosteam or vapor, is known as the latent heat ofvaporization. The latent heat of vaporization islost to the engine , since the exhausttemperature is always above the dew engine has no opportunity to convert thisheat into work. The amount of heat that is leftover for the engine to convert to work iscalled the low heat valueof the fuel (alsoreferred to as the net heat value). Low heatvalue can be calculated as the high heat valueminus the latent heat of NumberMany gases, including natural gas, landfillgas, digester gas, propane, etc.

6 Can beeffectively used in Caterpillar Gas gas compositions require differentcompression ratios and ignition timings, ormay require that the engine be derated. Somefuels may not be usable at , over the years, has used a numberof approaches to analyze gaseous fuels todetermine their suitability for combustion inreciprocating Engines . One of the firstmethods used was the Octane Ratingmethod,which indicates the knock resistance of agaseous fuel. This was adapted by the gasengine industr y from petroleum reciprocatingengine technology and compared unknowngaseous fuels with liquid reference methodology multiplies the percentagemole volume of each constituent in a gas byits Octane Ratingnumber and then sumsthese values, (obtained from comparing theindividual component gases to octane). Itincorrectly assumes the octane contributionfor the constituents is linear. This method alsodoes not take into account constituents withknock resistance characteristics, such ascarbon the past, the Octane Ratingmethod hasbeen an acceptable fuel analysis when appliedto pipeline and similar gases.

7 With today sgrowing market opportunities and widerrange of gases available, it has limited usesand restricts the Engines to known gas engine manufacturers still use theOctane Ratingmethod to analyze gaseousfuels despite these shortcomings. A morereliable method was needed to evaluategaseous the mid 1980s, Caterpillar adopted theMethane Numberapproach for analyzinggases in research and development work. Wefound good results and consistent engineperformance on a much broader range ofgases than the Octane Rating Methane Numberanalogy was developedin Austria in the mid 1960s. It compares theunknown resistance to knock of gaseous fuelwith the knock resistance of gaseousreference fuel. Using two reference gases,methane with greatest resistance to knockcharacteristics and hydrogen as the knock-prone component, a methane number can beassigned to any gaseous fuel. This is achievedby matching the knock characteristics of theunknown gaseous mixture to the knockcharacteristics of a blend of the two referencegases.

8 The percentage of methane in thereference gas mixture is the methane numberof the unknown extensive research and testing on fieldgases to landfill gases, Caterpillar has foundthe Methane Numberanalogy to be anaccurate and reliable assessment whenanalyzing fuels. Caterpillar considers it themost advanced technology in this field. It hasproven to be a clear competitive of the Methane Numberis difficultand time consuming. An approximationmethod was developed called the CaterpillarGas Valuenumber. The method wasdeveloped only for the G3408 and G3412 Engines . It was limited to certain gases andsimilar to the Octane Ratingmethod, did nottake into account fuels that have knockresistance constituents. In 1989 Caterpillardeveloped a computer program to performthe calculations and allow field determinationof the Methane program inputsgaseous constituents from the sample takenfrom the supply gas for the engine , andcalculates the methane number, the LHV, thewobbe index, and the relative power capabilitycompared to MJ/Nm3(905 Btu/ft3) fuel.

9 6 Methane numbers of some individualcomponent gases (commercial) .. calculating the Methane Numberandknowing the aftercooler water temperature(or Air-to-Air AfterCooling temperature)available, the engine rating can be determinedfrom the fuel usage guides published byCaterpillar. The guides show engine powerand timing for specified ranges of methanenumber for each aftercooler Required for CombustionAs indicated by Figure 1, each combustiblegas requires a definite volume of air forcomplete combustion of a given volume of thegas. This exact amount of air combined with agiven amount of gas is called thestoichemetric air-fuel ratio (or chemicallycorrect air-fuel ratio). There is a chemicallycorrect air-fuel ratio for each gas. This ratiovaries for the different gases. Anunderstanding and working knowledge of thispart of the chemistr y of combustion is asimportant to the Application engineer as to thedesign engineer.

10 This will become evidentlater in the determine the minimum amount of airrequired for complete combustion, refer tothe combustion equation for methane:CH4+ 2 O2= 2 H2O + are interested in the volume of O2and, inturn, the volume of air the required O2represents. The coefficients in thecombustion equation (the number ofmolecules) give the combining volumes of thegaseous components. Thus, one Ft3( ) of CH4requires two Ft3( ) of O2. Since air is of O2byvolume, the 2 Ft3( ) of O2represents:2 = air; = is the air required theoretically forcomplete combustion of one Ft3( ) ofCH4. A little excess air is usually provided formost gases to ensure complete combustion. The same results derived here can also bedetermined using the molecular weights tofirst determine the weight of air required,then converting the weight of air to volume ofair. The volume method is less stated earlier, most fuel gases are mixturesof several gases.