Environmental impact of shipping engines can be higher on a port-specific basis. Ultra-low sulfur diesel (ULSD) is a standard for defining diesel fuel with substantially lowered sulfur contents. As of 2006, almost all of the petroleum-based diesel fuel available in Europe and North America is of a ULSD type. In 2016 the IMO has made new sulfur regulations which must be implemented by larger ships by 2020. Of total global air emissions, shipping accounts for 18 to 30 percent of the nitrogen oxide and 9 percent of the sulphur oxides. Sulfur in the air creates acid rain which damages crops and buildings. When inhaled,
Underground coal gasification oxidants (air, oxygen) and steam to ignite and fuel the underground combustion process. Separate production wells are used to bring the product gas to the surface. The high pressure combustion is conducted at temperature of , but it may reach up to . The process decomposes coal and generates carbon dioxide (), hydrogen (), carbon monoxide (CO), methane (). In addition, there are small quantities of various contaminants including sulfur oxides (), mono-nitrogen oxides (), and hydrogen sulfide (). As the coal face burns and the immediate area is depleted, the oxidants injected are controlled by the operator. There are