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by the natural gas input.  The carbon efficiency is the carbon in the fuel divided by the carbon in the feedstock (not counting natural gas for compressors).  Higher heating values are used to represent energy efficiency in the United States and lower heating values are used in Europe.  Efficiency values in this report are identified at HHVs.

Combustion emissions from FTD and methanol plants are purged synthesis gas. Since the purge gas consists primarily of hydrogen, CO2 and CO with low levels of CH4 and ppm levels of HCs, NMOG emissions from reformers are extremely low.  Emissions estimates for FTD production are shown in Table 4-18.

Steam reforming results in an excess of hydrogen for each mole of carbon.  In steam reforming systems, the purge gas provides fuel to the reformer.  Purge gas input to the reformer exceeds the energy requirements of the reformer for generating steam and the reforming reaction.  Excess steam energy can be used to power compressors or generate electric power.

The subject of steam export and credits for steam exports is a key issue for fuel-cycle studies.  Credit for steam production or electric power generation can be given for export steam.  Several approaches exist for providing credits for excess process energy.  The energy required to generate steam in a boiler from natural gas can be determined and used as a credit, primarily for process energy and CO2.  Also, the credit can be calculated in terms of energy required to generate electric power. Steam can only be exported to adjacent facilities the plant is collocated with an oil refinery or chemical plant that can utilize the steam.  The subject of credits for excess steam can have a significant impact on the CO2 balance.  Steam is produced from excess process heat and by burning hydrogen and has a very low CO2 emissions associated with it as the carbon efficiency of methanol and FTD production is relatively high.  If excess steam is credited with power generation from natural gas or coal, the resulting credit will increase the effective carbon efficiency of fuel production.

Table 4-18:  FTD Processing From Natural Gas

Syntroleum Process

Process

No Steam Export

Steam Export

Fuel Processing

Steam export (Btu/gal)

0

32,200

Energy ratio (%)

54

53

NG feed (Btu/gal)

 (100 scf/gal)

238,000

2.31

242.500

2.35

Emission Source

Fugitive

Vent

Fugitive

Vent

Emissions (g/gal)

 NOx

0

0.1

0

0.1

 CO

0

0.50

0

0.5

 CH4

3

0.04

3

0.04

52

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