Fuel Equivalents

1. Purpose

This procedure describes the Inflection Point Engineering practice for calculating the Fuel Equivalent (FE) of a utility. Fuel Equivalents put all forms of energy on a common basis, taking into account both thermodynamic and mechanical efficiencies. The Fuel Equivalent shall be used as a guide in determining the tradeoff among the various utilities.

2. Definition of Fuel Equivalents

Fuel Equivalent is defined as the amount of fuel energy (typically the lower heating value expressed in BTU's or equivalent) required to make an amount of utility. Thus,

The methods for calculating the Fuel Equivalents for utilities at specific refineries are described below. For quick reference, Table 1 lists FE's for utilities at a typical refinery, assuming typical efficiencies.

Table 1 - Fuel Equivalents for Refinery Utilities

3. Methods for Calculating Fuel Equivalents

3.1 Fuel

There are several types of fuel in a refinery. Typical fuels include fuel gas, natural gas, fuel oil, and LPG. Less often seen are solid fuels such as coke and other liquid fuels such as kerosene or diesel. For all fuels, the Fuel Equivalent is equal to one.

FE Fuel = 1 BTU per BTU

3.2 Power

Electric power can be produced onsite or purchased from an offsite source. Based on Solomon surveys, it is assumed that purchased electrical power is produced using 9,090 BTU per kw-hr. This is reflective of a typical utility power plant with an efficiency of about 38%. The Fuel Equivalent of imported power then is:

FE Power = 9,090 BTU per kw-hr

With onsite power generation using condensing steam turbine generators, calculate the Fuel Equivalent with the following:

FE Power = (3,414 BTU per kw-hr ) / cycle

If unknown, assume a cycle efficiency of 30%, yielding an FE Power of 11,500 BTU per kw-hr.

When onsite power is generated by a gas turbine generator (GTG), calculate the FE after allocating fuel costs between the GTG and the accompanying heat recovery steam generator (HRSG). It is typical to assume that the amount of fuel ‘consumed’ in the HRSG is the same as that required to produce the same amount of steam in the utility boiler as is produced in the HRSG. The fuel used by the GTG is then defined by Fuel Chargeable to Power (FCP). Calculate FE Power with:

where FCP = Fuel Used in GTG/HRSG - Fuel Saved by Not Making Steam in Boiler

For a Frame 6 GTG, power production is nominally 40,000 kw and fired steam production is nominally 300,000 lb/hr. Total fuel consumption is about 600 MMTBU/hr. FE Power will then equal about 4,800 BTU per kw-hr.

3.3 Steam

A typical refinery has three steam headers, typically designated as high pressure, medium pressure, and low pressure. In general, the higher the pressure and temperature, the higher the Fuel Equivalent. The following provides FE’s for each level of steam. Use the rules for the medium pressure header for additional headers if there are more than three steam levels.

a. High Pressure Steam

High pressure steam is defined as the steam produced by the utility boilers. This is typically at the 600 psig / 750 F level, but is often at 1,500 psig / 950 F if power is generated onsite. Calculate the Fuel Equivalent of high pressure steam by dividing the difference in enthalpy between the boiler feedwater (BFW) and steam by the efficiency of the boiler (ignore boiler blowdown), and then adding on the FE of the BFW:

Boiler auxiliaries, such as fans, forced circulation pumps, etc, would be reflected in boiler efficiency. Assume a boiler efficiency of 85% if details are not available. With BFW at 250F, HP steam at 600 psig / 750 F will have an FE HP Steam equal to 1,565 BTU/lb of steam.

b. Medium & Low Pressure Steam

Medium and low pressure steam is produced by process waste heat steam generators, turbine exhausts, letdown stations, etc. MP steam is typically in the 125 to 250 psig range. LP steam is typically at 15 psig or 50 psig. Calculate the Fuel Equivalent of LP or MP steam with:

The theoretical steam rate (TSR) is a function of the turbine inlet and outlet steam conditions and should be obtained from an appropriate reference. Common TSR's, for steam at 750F, are below:

Turbine efficiencies will vary. Assume 70% on average for a large refinery if not otherwise known. For a 150 psig header, FE MP Steam will equal 1,300 BTU per lb of MP steam. For a 50 psig header, FE LP Steam will equal 1,150 BTU per lb of LP steam.

Low pressure steam is the lowest rung of the steam value ladder. A surplus can not be let down to a lower pressure, although the LP steam can be condensed. If the steam is usable, it is still of relatively high value; calculate FE LP Steam as above. If it is vented, it is of no value; any generated LP steam will have an FE LP Steam equal to 0. If the LP steam is condensed, value the LP steam as hot condensate (ignoring condenser utilities) and calculate FE with:

FE LP Steam = h LP Saturated Water - h Ambient Water

3.4 Condensate

Calculate the Fuel Equivalent assuming that LP steam is used to 'make' condensate.

With an ambient temperature of 60 F, and a condensate temperature of 200 F,

FE Condensate will equal about 140 BTU per lb of condensate.

3.5 Boiler Feedwater

Calculate BFW's Fuel Equivalent with the following, accounting for both BFW's heat content and the power used to elevate its pressure. The FE assumes LP steam is used to make BFW.

The specific gravity (SG) will typically be 0.94 for a 15 psig deaerator. For 600 psig steam, assume a pump P of 830 psi if not otherwise known. FE BFW will be equal to about 200 BTU per lb of 250 F BFW. Note that the FE of the pumping power is very low (~ 10 BTU/lb of BFW), even with the very high pump P.

3.6 Cooling Water

There are two main types of cooling water systems, once through and recirculating. In a recirculating system, calculate the Fuel Equivalent based on the power used in driving the pumps plus the power used in running the cooling tower fans.

Power for fans ranges from 4 to 20 kw per 1000 gpm of throughput. If unknown, assume the fan power equals 7 kw / 1000 gpm. Assume a pump P of 65 psi if not otherwise known. Using these values, FE Cooling Water will equal about 6.8 BTU per gallon of cooling water.

For a once through system, determine the FE using the above equation with a fan power of zero and the cooling water pumps' P. Assuming a P of 100 psi will yield an FE Cooling Water equal to about 8.85 BTU per gallon of cooling water.

3.7 Fresh Water

For the purposes of this procedure, we assume the FE of fresh water, as well as demineralized water, softened water, well water, etc. is negligible. We ignore pumping power. Thus,

FE Water = 0

Desalinated water can also given an FE of zero. Once the desalinated water is produced, it has essentially no heat content. However, desalination consumes a large amount of heat input as LP steam is condensed in the evaporation of sea water. Thus, calculate the Fuel Equivalent with:

FE Desalinated Water = lbs of LP steam per lb of water x FE LP Steam

Assuming 1 pound of LP steam is required to make 10 lbs of desalinated water,

FE Desalinated Water is equal to 115 BTU/lb.

3.8 Compressed Air

Calculate the Fuel Equivalent of plant air based on required compressor power. Ignore any cooling water requirements.

FE Plant Air = Power Requirements per SCF of air x FE Power

Assuming a centrifugal compressor with an electric motor driver using 210 kW of electricity / 1000 scfm, FE Plant Air will equal about 32 BTU per scf.

Instrument air (IA) is simply plant air that has been processed in air dryers to remove humidity. The instrument air FE should be calculated based on actual dryer utilities. For simplicity, calculate by assuming dryer utilities equal 10% of the plant air's FE.

= 1.1 x FE Plant Air = 35 BTU per scf

3.9 Nitrogen

Calculate the Fuel Equivalent of nitrogen based on the power used to produce the nitrogen.

FE Nitrogen = Power Requirements per SCF of nitrogen x FE Power

If unknown, assume 0.05 kw-hr per scf of gaseous nitrogen (e.g., if imported as a gas through a pipeline) or 0.1 kw-hr per scf of liquid nitrogen (e.g., if imported as liquid on a truck). FE Nitrogen will then equal 455 Btu per scf for gaseous nitrogen.