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The ash content of liquid fuels is important in connection with cleanliness，corrosion， and deposition characteristics of the fuel. Ash is the material remaining after combustion. Ash is present in two forms: (1) as solid particles correspondingto that material called sediment， and (2) as oil or water soluble traces ofmetallic elements. As mentionedearlier， sediment is a measure of cleanliness. The corrosivity of a fuel is related to the amount of various trace elements in the fuel ash. Certain high-ash fuels tend to be verycorrosive. Finally， since ash is the fuel element remaining after combustion， the deposition rate is directly related to the ash content of the fuel.
Table 12-3 is a summary of gaseous fuel specifications. The two major areas of concern are heating value with its possible variation and contam-inants. Fuels outside a specification can be utilized if some modification is made.
Gas fuels with heating values between 300-1100 Btu/ft3(11，184-41，000 KI/m 3)are in use today; however， future systems may use gas with heating values
Table 12-3
Gaseous Fuel Specifications

Heating value 300-1100 Btu/ft3(11，184-41，000 KI/m 3) Solid contaminants . 30ppm Flammability limits 2.2:1Composition.S，Na，K， Li . 5ppm .Sulfur十 sodium十 potassium十 lithium. (When formed into alkali metasulfate) H2O (by weight) . 25.
below 100 Btu/ft3(3728 KI/m3). Although wide ranges of heating values canbe accommodated with different fuelsystems， the maximum variation that can be used in a given fuel system is .10%.
Sulfur content must be controlled in units with exhaust recovery systems.If sulfur condenses in the exhauststack， corrosion can result. In units with-out exhaust recovery there is noproblem， since stack temperatures areconsiderably higher than the dew point. Sulfurcan， however， promote hot-section corrosion in combustion with certain alkali metals such as sodium or potasium. This type of corrosion is sulfidation or hot corrosion and is controlled by limiting the intake of sulfur and alkali metals. Contaminants found in a gas depend on the particular gas. Common contaminants includetar， lampblack， coke， sand， and lube oil.
Table 12-4 is a summary of liquid fuel specifications set by manufacturers for efficient machine operations. The water and sediment limit is set at 1% by maximum volume to prevent fouling of the fuel system and obstruction of the fuel filters. Viscosity is limited to 20 centistokes at the fuel nozzles toprevent clogging of the fuel lines. Also， it is advisable that the pour point be 20 0F (11 0C) below the minimum ambient temperature. Failure to meet this specification can be corrected by heating the fuel lines. Carbon residue should be less than 1% by weight based on 100% of the sample. The hydrogen content is related to the smoking tendency of a fuel. Lower
Table 12-4
Liquid Fuel Specifications

Water and sediment  1.0% (V%) Max. 
Viscosity  20 centistokes at fuel nozzle 
Pour point  About 20 0 below min. ambient 
Carbon residue  1.0% (wt) based on 100% of sample 
Hydrogen  11.% (wt) minimum 
Sulfur  1% (wt) maximum 

Typical Ash Analysis and Specifications
Metal  Lead  Calcium  Sodium & Potassium  .anadium 
Spec. max. (ppm)  1  10  1  0.5 untreated 
Naphtha  0-1  0-1  0-1  500 treated 
Kerosene  0-1  0-1  0-1  0-.1 
Light distill.  0-1  0-1  0-1  0-.1 
Heavy distill. (true)  0-1  0-1  0-1  0-.1 
Heavy distill. (blend)  0-1  0-5  0-20  .1/80 
Residual  0-1  0-20  0-100  5/400 
Crude  0-1  0-20  0-122  .1/80 

hydrogen-content fuels emit more smoke than the higher-hydrogen fuels. The sulfur standard is to protect from corrosion those systems with exhaust heat recovery.
　
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