Gas Engines Application and Installation Guide G3600–G3300 ● Fuels ● Fuel Systems LEKQ7256 (Supersedes LEKQ2461) 10-97 G3600–G3300 Fuels Fuel Characteristics Hydrocarbons Standard Condition of a Gas Heat Value Methane Number Air Required for Combustion Common Fuels Natural Gas Sour Gas Propane Propane-Butane Mixtures Propane-Air Propane Fuel Consumption Calculations Digester Gas Sanitary Landfill Gas Manufactured Gases Constituents of Gas by Volume - Percent Producer Gas Illuminating Gas Coke-Oven Gas Blast Furnace Gas Wood Gas Cleaning Fuel Effects on Engine Performance Heat Value of the Air-Fuel Mixture Turbocharged Engines Methane Number Program Calculations Fuel Consumption Detonation Methane Number Compression Ratio Ignition Timing Load Inlet Air Temperature Air-Fuel Ratio Emissions Variations in Heating Value Fuel Temperature Recommendations Fuel Requirements Heating Value Fuels As the number of atoms increases, the molecular weight of the molecule increases and the hydrocarbons are said to become Most of the fuels used in internal combustion heavier. Their physical characteristics change engines today, whether liquid or gaseous, are with each change in molecular structure. composed primarily of hydrocarbons Only the first four of the Paraffin series are (hydrogen and carbon); their source is considered gases at standard conditions of generally petroleum. Natural gas is the most 101.31 kPa (14.696 psia) and 15.55°C (60°F). popular and widely used of the petroleum Several of the others can be easily converted gases. Digester gas (also a hydrocarbon) and to gas by applying a small amount of heat. some manufactured gases (from coal), which contain hydrocarbons, are also used in Standard Condition of a Gas engines with varying degrees of success. It is important to note that when standard Digester gas is the most practical of the conditions are referenced, it means manufactured or by-product group. 101.31 kPa (14.696 psia) and 15.55°C (60°F). When a gaseous fuel flow is stated in SCF, it Each commercial fuel gas is a mixture of means standard cubic feet (or standard cubic gases, some combustible and some inert. The meters - SCM) and is referenced to a gas at different mixtures have extremely wide standard conditions. In some places, Europe variations in composition. Consequently, it is for example, gas is referenced to 101.31 kPa necessary to closely examine the (14.696 psia) and 0°C (32°F). When gases are characteristics and behavior of an individual referenced to 0°C (32°F), the units are called gas. normal cubic meters (NM3) or normal cubic feet (NF3). Heat Value Fuel Characteristics Heat value is defined as the amount of energy Hydrocarbons (heat) released during the combustion of a Hydrocarbons are grouped into three fuel with the correct amount of oxygen (air). classifications according to their molecular It is determined with a device called a structure. calorimeter. A known quantity of fuel and oxygen are combined in a calorimeter and • Paraffins - CnH2n+2 burned. Heat is generated and, the water • Napthenes - CnH2n produced (from the combustion of fuels • Aromatics - CnH2n-6 containing Hydrogen; either Cx Hy or H2) is Most of the important fuel gases used in condensed. The heat measured by the engines today are of the Paraffin series. This calorimeter is called the high heat value of the includes both natural gas and digester gas. fuel (also referred to as the gross heat value). This series starts with methane (CH4); each succeeding member of the series has one It is important to understand the difference more carbon (C) atom and the corresponding between high and low heat value since engine number of hydrogen (H) atoms, etc. The manufacturers typically use low heat value normal Paraffin hydrocarbons are said to when discussing fuels and engine data. A have straight chain molecular structures, discussion of heat value as it relates to the having one bond between each atom. The first internal combustion engine may help provide four of the Paraffin series would have a better understanding of the difference structures as follows: between high and low heat value. When any hydrocarbon is used as fuel in an H H H H H H H H H H internal combustion engine, one of the H-C-H H-C-C-H H-C-C-C-H H-C-C-C-C-H products of combustion is water. The amount H H H H H H H H H H of water formed during combustion varies for Methane Ethane Propane Butane the different hydrocarbon fuels. This will be CH4 C2H6 C3H8 C4H10 illustrated later. The water formed is converted into steam by the combustion heat 5 before leaving the engine, and carries with it Octane Rating method to analyze gaseous the quantity of heat used to convert the water fuels despite these shortcomings. A more into steam. This quantity of heat absorbed in reliable method was needed to evaluate changing water, at a given temperature to gaseous fuels. steam or vapor, is known as the latent heat of vaporization. The latent heat of vaporization is In the mid 1980s, Caterpillar adopted the lost to the engine, since the exhaust Methane Number approach for analyzing temperature is always above the dew point. gases in research and development work. We The engine has no opportunity to convert this found good results and consistent engine heat into work. The amount of heat that is left performance on a much broader range of over for the engine to convert to work is gases than the Octane Rating method. called the low heat value of the fuel (also The Methane Number analogy was developed referred to as the net heat value). Low heat in Austria in the mid 1960s. It compares the value can be calculated as the high heat value unknown resistance to knock of gaseous fuel minus the latent heat of vaporization. with the knock resistance of gaseous Methane Number reference fuel. Using two reference gases, Many gases, including natural gas, landfill methane with greatest resistance to knock gas, digester gas, propane, etc. can be characteristics and hydrogen as the knock- effectively used in Caterpillar Gas Engines. prone component, a methane number can be Different gas compositions require different assigned to any gaseous fuel. This is achieved compression ratios and ignition timings, or by matching the knock characteristics of the may require that the engine be derated. Some unknown gaseous mixture to the knock fuels may not be usable at all. characteristics of a blend of the two reference gases. The percentage of methane in the Caterpillar, over the years, has used a number reference gas mixture is the methane number of approaches to analyze gaseous fuels to of the unknown gas. determine their suitability for combustion in reciprocating engines. One of the first After extensive research and testing on field methods used was the Octane Rating method, gases to landfill gases, Caterpillar has found which indicates the knock resistance of a the Methane Number analogy to be an gaseous fuel. This was adapted by the gas accurate and reliable assessment when engine industry from petroleum reciprocating analyzing fuels. Caterpillar considers it the engine technology and compared unknown most advanced technology in this field. It has gaseous fuels with liquid reference fuels. proven to be a clear competitive advantage. The methodology multiplies the percentage Calculation of the Methane Number is difficult mole volume of each constituent in a gas by and time consuming. An approximation its Octane Rating number and then sums method was developed called the Caterpillar these values, (obtained from comparing the Gas Value number. The method was individual component gases to octane). It developed only for the G3408 and G3412 incorrectly assumes the octane contribution Engines. It was limited to certain gases and for the constituents is linear. This method also similar to the Octane Rating method, did not does not take into account constituents with take into account fuels that have knock knock resistance characteristics, such as resistance constituents. In 1989 Caterpillar carbon dioxide. developed a computer program to perform the calculations and allow field determination In the past, the Octane Rating method has of the Methane Number. The program inputs been an acceptable fuel analysis when applied gaseous constituents from the sample taken to pipeline and similar gases. With today’s from the supply gas for the engine, and growing market opportunities and wider calculates the methane number, the LHV, the range of gases available, it has limited uses wobbe index, and the relative power capability and restricts the engines to known gases. compared to 35.6 MJ/Nm3 (905 Btu/ft3) fuel. Some gas engine manufacturers still use the 6 Methane numbers of some individual This is the air required theoretically for component gases are: complete combustion of one Ft3 (0.0283m3) of CH4. A little excess air is usually provided for Methane CH ...........................................100 4 most gases to ensure complete combustion. Ethane C2H6............................................44 Propane C3H8............................................32 The same results derived here can also be Butane (commercial) .............................15 determined using the molecular weights to n-Butane C4H10 ..........................................10 first determine the weight of air required, Hydrogen H ..................................................0 2 then converting the weight of air to volume of After calculating the Methane Number and air. The volume method is less complex. knowing the aftercooler water temperature As stated earlier, most fuel gases are mixtures (or Air-to-Air AfterCooling temperature) of several gases. Each component gas has available, the engine rating can be determined different characteristics. Determination of the from the fuel usage guides published by amount of air required per unit volume of a Caterpillar. The guides show engine power mixture of gases requires that the different and timing for specified ranges of methane characteristics of components be recognized.