Compressed Natural Gas (CNG)
Compressed natural gas, or CNG, is produced by compressing NG down to less than 1% of its volume at standard atmospheric pressure. The CNG volume to be transported or stored can be from 150 to 300 times less than gas at atmospheric pressure. Producing CNG is a simple low-cost process and involves gas pre-treatment (depending on the gas source and quality) and compression. The NG arrives at the compression station at a low pressure from a local pipeline or truck, where it is compressed and stored in cylindrical or spherical storage containers at a pressure of 100 bar to 250 bar.
The main function of CNG is as an alternative for gasoline and diesel fuels for medium-duty vehicles travelling a moderate distance between refuelling. There are some other uses of CNG that include power generation and industrial consumers, however these are less developed due to the gas volumes required often being too high. CNG is typically used onshore for gas supply over short distances and in smaller volumes. Generally, CNG can be economically viable for volumes up to 140 000 ms3/day (5 million standard cubic feet per day (MMscf/d)), and distances up to 800 km.
The transportation of CNG can be onshore by truck or offshore by ship or barge. The largest cost in the CNG supply chain is the midstream transportation component, contributing up to 90% of the capital required. CNG onshore transportation by truck involves the facilities to load CNG into a pressurised transportable container at the compression site and the offloading facilities that includes the heating, let down and metering of the CNG at the customer site. The CNG offshore transportation by ship or barge involves a vessel with a containment system to transport the CNG from the source to the customer site. Figure 4 shows some of the different designs of trucks, ISO containers, and marine vessels for the transport of CNG.