GC: n

CT: Rich gas and lean gas are the two types of natural gas that are distributed in Belgium via different networks. The difference between them is their calorific value, i.e. a different quantity of energy. In other words, 1 m³ of lean gas provides less energy than 1 m³ of rich gas.
However, this difference does not have any impact on your bill, as your supplier bases this on the quantity of energy that you have actually used and not on the quantity of gas supplied to you

S: http://www.energuide.be/en/questions-answers/what-is-the-difference-between-rich-and-lean-gas-does-this-have-an-impact-on-my-bill/85 (last access: 19 December 2014)

N: 1. lean (adj): “thin, spare, with little flesh or fat,” c.1200, from Old English hlæne “lean, thin,” possibly from hlænan “cause to lean or bend,” from Proto-Germanic khlainijan, which would connect it to Old English hleonian. But perhaps rather, according to Oxford English Dictionary, from a PIE qloinio-. Extended and figurative senses from early 14c. The noun meaning “lean animals or persons” is from c.1200, from the adjective.
gas (n): 1650s, from Dutch gas, probably from Greek khaos “empty space”. The sound of Dutch “g” is roughly equivalent to that of Greek “kh.” First used by Flemish chemist J.B. van Helmont (1577-1644), probably influenced by Paracelsus, who used khaos in an occult sense of “proper elements of spirits” or “ultra-rarified water,” which was van Helmont’s definition of gas.
Modern scientific sense began 1779, with later specialization to “combustible mix of vapors” (1794, originally coal gas); “anesthetic” (1894, originally nitrous oxide); and “poison gas” (1900). As short for gasoline, it is American English, first recorded 1905.
2. Residual gas, mainly methane and ethane, that remains after the heavier hydrocarbons have been condensed in the wellhead. When the lean gas is liquefied, it is called liquefied natural gas (LNG).
3. Rich gas contains 5 or 6 gallons or more of recoverable liquid hydrocarbons per thousand cubic feet. Whereas Lean gas contains less than 1 gallon of recoverable liquid hydrocarbons per thousand cubic feet.

S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=lean&searchmode=none; http://www.etymonline.com/index.php?allowed_in_frame=0&search=gas&searchmode=none (last access: 19 December 2014). 2. http://www.glossary.oilfield.slb.com/en/Terms/l/lean_gas.aspx (last access: 19 December 2014). 3. http://www.hindustanpetroleum.com/cng#a3 (last access: 19 December 2014).

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CR: biomass, biomass energy, gasification, gasifier.