Compressed Natural Gas

The natural gas used is extracted in Algeria and transported through pipeline to the STCP depot. According to [4], extraction efficiency is 99,4%.

Natural gas processing includes removal of contaminants as C02 and heavy hydrocarbons. In the process leaks occur, and this adds to the total emissions of C02 equivalent.

The natural gas is transported by pipeline, at a pressure of ca. 70 bar, through Algeria, North Africa up to Tangiers, where it crosses near Gibraltar to Spain. Inside Portugal, the gas is distributed in the main network at a pressure of 70 bar. The gas is considered to reach the final customer at a pressure of 7 bar. The total distance is assumed to be 1250 km. Some natural gas is used as fuel in the intermediate compression stations, the overall efficiency is assumed to be 97,8% according to [3].

The natural gas vehicles of STCP store the natural gas onboard at a pressure of 200 bar, so a compression stage is needed. The final pressure is 250 bar in order to guarantee enough pressure differential to fill the tanks.

Подпись: n-R-T, к — 1 1 Compressed Natural Gas

The work needed for an isentropic compression can be expressed as

where: k = adiabatic exponent

n = number of moles T1 = temperature at pi R = Perfect gas constant = 8,314 J/(mol. K) pi = inlet pressure p2 = output pressure (filling)

The isentropic efficiency of the process is considered to be 68 % [4]. The final energy consumption (electricity) for the filling process is 0,008 kWhel / MJ of CNG.

The global result for natural gas processing is presented on Table 11.

Table 11. Energy consumption, losses and GHG emission for natural gas production.

Energy

supplied

Energy losses

GHG emission

(MJ/MJN0)

(MJ/MJNG)

%

(g/MJNG)

%

Extraction of natural gas

1,065

0,006

9,2

0,62

6,8

Processing of NG

1,059

0,007

10,8

0,61

6,7

Transport of NG

1,052

0,023

35,4

2,53

27,7

Compression/Filling

1,029

0,029

44,6

5,37

58,8

Total

1,065

0,065

100

9,13

100

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