Tham khảo tài liệu 'biofuels, solar and wind as renewable energy systems_benefits and risks episode 2 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 11 Our Food and Fuel Future 263 the wellhead. During the 1970s price rose from 17 cents to per thousand cubic feet and during the 1980s and 1990s natural gas was irregularly priced but sometimes above . A substantial price rise to 2007 levels fluctuating between 5 and 7 per thousand cubic feet began about the year 2000. Improved technologies of horizontal drilling and fracturing in tight rock formations have enabled gas production in areas of shale and coal formations in the United States and the high cost of production is supported by high price of the product. Regrettably modern methods of extraction often degrade soil and water. Natural gas is widely used today for home heating and for standby power generation and gas-to-liquids technologies are being proposed for production of liquid fuels. Gas production and consumption in the United States has been nearly steady at about 24 trillion cubic feet annually since the mid-1990s and challenges to maintain that level of usage in the presence of an ultimate decline of . supplies have led to proposals for importation of liquefied strongly cooled gas LNG from the Middle East. However proposed LNG terminals are often opposed by local groups apprehensive of explosion dangers. Natural gas is also used for production of the fertilizer bases ammonium nitrate and urea. As the price of natural gas has risen its preferred use for home heating and power generating facilities has led to closure of about 40 of . fertilizer production capacity since 1999 and to increasing importation of nitrogen fertilizer from regions where natural gas is much less costly than in the . Imports now account for a little more than half of total . nitrogen supply which has remained nearly steady at twenty million product tons since 1998. A recently developed controversy within the United States involves proposed new facilities for electric power generation with natural gas interests pointing to the lower carbon dioxide emissions .
