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The calculations estimated that emissions associated with the LPG production are proportional to the quantity of LPG produced.  Thus 3.7 percent of the methane and CO2 emissions during the production process could be accounted in an LPG fuel-cycle analysis.  Although one could claim that the emissions are a result of natural gas production since LPG contains unwanted natural gas components, the marketability of LPG requires accountability for the emissions.  It is also useful to note that without data to describe the exact losses due to each natural gas component at each stage of processing, it is difficult to calculate the specific emissions.  However, it is sufficient to extrapolate from the masses of methane, NMOG, and LPG in the end products.

4.4  Synthetic Diesel Production

Natural gas is converted to synthesis gas by reforming the feedstock with steam and oxygen.  Natural gas is the simplest feedstock to convert to synthesis gas since it is gas and does not need to be processed in a gasifier.  This synthesis gas is over 90 percent carbon monoxide and hydrogen with traces of methane and nitrogen.  The FT reactor uses iron or cobalt catalysts in a fluidized bed reactor.  Excess heat from the FT reactor produces steam for the reformer.  Additional thermal energy can be used to generate steam to produce electric power or provide other process heat requirements such as powering desalinization plants.  Wax is converted to liquid fuels by reacting with hydrogen in the final step of the process.  The energy ratio (fuel output/feedstock input) for a natural gas to FT diesel plant is about 56 percent (HHV basis).  This value does not include uses for excess thermal energy.

A FTD plant consists of the following processes:

Synthesis gas production (reforming and partial oxidation)

Catalytic hydrocarbon production

Final product separation

Emissions from FTD diesel facilities were estimated as either combustion emissions or fugitive emissions.

Understanding the configuration of synthetic fuels plants helps illustrate the fate of carbon and net CO2 as well as combustion and compressor engine emissions.  A synthetic fuels plant with a steam reformer and POX reactor is illustrated in Figure 4-2.  A steam reformer converts steam and CH4 to CO and hydrogen.  An excess of hydrogen is produced with a steam reformer.  The CO and hydrogen mixture flows over a catalyst where methanol or other fuels are produced.  This reaction occurs at high pressures (30 atm) as equilibrium does not favor synthesizing methanol or hydrocarbons at low pressure.  The synthesis gas may be recirculated several times over the catalyst or flow over the catalyst in a single pass (once through process).  Recirculating the synthesis gas results in a higher fuel conversion rate.  The power required to compress and circulate


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