1. Home page of The Japan Gas Association, 1-15-12, Toranomon, Tokyo, http://www.gas.or.facts/genryo.html
2. A. Obuchi, H. Taniguchi, SNG plant used membrane for CO2 removal, The Piping Engineering, 86–90, July 1995, 6-3-26, Komagome, Tokyo; htt.//www.takarabune.or.jp/nikko/
3. Technical instruction for high calorie city gas production process, June 1994, The Japan Gas Association, See [1]
4. R.W. Spillman, M.G. Barrett, T.E. Cooley, Gas Membrane Process Optimization, AlChE Meeting in New Oreleans, 9 March 1988
5. A. Obuchi, H. Taniguchi, City gas production, JP Patent No. 2584716 (other related patents and publications)