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Assuming that natural gas is the base line fuel to obtain the same power using diesel fuel the gas turbine would have to be fired at a higher tempera-ture， and for low Btu (400 Btu/cuft， 14911 KI/m 3) gases at the same firing temperature the turbine would produce more power due to the fact that theamount of fuel could be increased bythreefold， thus increasing the overall mass flow through the turbine. The limitation in using low Btu gases is that it takes about 30% of the air for combustion as compared to 10% of the air for natural gas leaving much less air for cooling the combustor liners. Because of this for low Btu gases it is easier to modify annularcombustor turbines， which have less of a combustor liner surface area than can-annular combustors. Another problem is that in some cases the extra flow can choke the turbine nozzles. For turbines used in combined cycle

Figure 12-1. Effec. o. .arious fue.s on turbine in.e. temperature.
application there is a tendency to keep the same firing temperature at off-load conditions but with the use of the inlet guide vanes vary the airflow rate.
Fuel Specifications
To decide which fuel to use， a host of factors must be considered. The object is to obtainhigh efficiency， minimumdowntime， and the total economic picture. The following are some fuel requirements that are important in designing a combustion system and any necessary fuel treatment equipment:
1. Heating value

2. Cleanliness

3. Corrosivity

4. Deposition and fouling tendencies

5. Availability

The heating of a fuel affects the overall size of the fuel system. Generally， fuel heating is a more important concern in connection with gaseousfuels， since liquid fuels all come from petroleum crude and show narrowheating-value variations. Gaseousfuels， on the other hand， can vary from 1100 Btu/ft3(41，000 KI/m 3) for natural gas to(11，184 KI/m 3) or below for process gas. The fuel system will of necessity have to be larger for theprocessgas， since more is required for the same temperature rise.
Cleanliness of the fuel must be monitored if the fuel is naturally ""dirty"" or can pick up contaminants during transportation. The nature of the con-taminants depends on the particular fuel. The definition of cleanliness here concerns particulates that can be strained out and is not concerned with soluble contaminants. These contaminants can cause damage or fouling in the fuel system and result in poor combustion.
Corrosion by the fuel usually occurs in the hot section of theengine， either in the combustor or the turbine blading. Corrosion is related to the amounts of certain heavy metals in the fuel. Fuel corrosivity can be greatly reduced by specific treatments discussed later in this chapter.
Deposition and fouling can occur in the fuel system and in the hot section of the turbine. Deposition rates depend on the amounts of certain com-pounds contained in the fuel. Some compounds that cause deposits can be removed by fuel treating.
Finally， fuel availability must be considered. If future reserves areunknown， or seasonal variations are expected， dual fuel capability must be considered.
Fuel requirements are defined by various fuel properties. By coincidence， the heating-value requirement is also a property and needs no further mention.
Cleanliness is a measure of the water and sediment and the particulate content. Water and sediment arefound primarily in liquidfuels， while particulates are found in gaseous fuels. Particulates and sediments cause clogging of fuel filters. Water leads to oxidation in the fuel system and poor combustion. A fuel can be cleaned by filtration.
Carbon residue， pourpoint， and viscosity are important properties in relation to deposition and fouling. Carbon residue is found by burning a fuel sample and weighing the amount of carbon left. The carbon residue property shows the tendency of a fuel to deposit carbon on the fuel nozzles and combustion liner. Pour point is the lowest temperature at which a fuel can be poured by gravitational action. Viscosity is related to the pressure loss in pipe flow. Both pour point and viscosity measure the tendency of a fuel tofoul the fuel system. Sometimes， heating of the fuel system and piping is necessary to assure a proper flow.
　
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