Tail gas is the residual gaseous effluent stream resulting from high-volume industrial processes. It is released after initial attempts have been made to remove the main pollutants and recover valuable products. It represents the final exhaust stream that contains trace amounts of unrecovered compounds and reaction byproducts. This gas is a primary focus of environmental regulation because of its persistent contaminant load, even after the majority of the process gas has been cleaned. The composition of this gas is highly variable, making its management a complex and costly challenge for manufacturers.
How Tail Gas is Generated
Tail gas is primarily a byproduct of large-scale chemical processing operations designed to purify raw materials or create new substances. The oil and gas refining sector is a major source, particularly from the Sulfur Recovery Unit (SRU), which converts highly toxic hydrogen sulfide (\(\text{H}_2\text{S}\)) into marketable elemental sulfur. The gas stream exiting the SRU is defined as tail gas because complete conversion is chemically and economically impractical within the primary unit itself. This residual stream is also generated during the regeneration of chemical solvents used to treat process streams, such as the amine solutions used to ‘sweeten’ natural gas by stripping out acidic components. Furthermore, industrial operations like nitric acid production create tail gas containing nitrogen compounds after the main absorption columns.
Defining the Chemical Makeup
The specific chemical composition of tail gas is entirely dependent on the upstream process from which it originates. In sulfur recovery operations, the gas is predominantly composed of inert gases like nitrogen and carbon dioxide (\(\text{CO}_2\)), along with water vapor. However, it is the trace contaminants that pose the environmental risk. These contaminants include unrecovered hydrogen sulfide (\(\text{H}_2\text{S}\)) and sulfur dioxide (\(\text{SO}_2\)), which are the primary sulfur-bearing pollutants remaining after the Claus process. Other harmful sulfur compounds are also frequently present, such as carbonyl sulfide (\(\text{COS}\)) and carbon disulfide (\(\text{CS}_2\)), which form as side reactions. Tail gas from other industries may contain high levels of nitrogen oxides (\(\text{NO}_x\)) and carbon monoxide (\(\text{CO}\)). Residual hydrocarbons and other volatile organic compounds (VOCs) may also be present, especially in refining and petrochemical tail gas streams.
Environmental Significance
The presence of these compounds makes the uncontrolled release of tail gas a significant environmental concern, directly contributing to several forms of pollution. Sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)) are precursors to acid deposition and the formation of ground-level ozone, a primary component of smog. These pollutants can harm sensitive ecosystems, damage plant life, and degrade infrastructure. Residual hydrogen sulfide (\(\text{H}_2\text{S}\)) is acutely toxic to humans and animals, even at low concentrations, posing an immediate health hazard to workers and nearby communities. The presence of residual hydrocarbons and carbon monoxide (\(\text{CO}\)) means the gas can also contribute to greenhouse gas emissions. National and international air quality standards mandate strict limits on the concentration of these substances, necessitating further treatment before the gas can be vented.
Required Treatment and Abatement
To comply with stringent emission regulations, industries must subject tail gas to a secondary cleaning process before it can be released into the atmosphere. This is typically accomplished using specialized Tail Gas Treating Units (TGTUs), which are often integrated directly after the primary recovery process. One common approach is to use catalytic reduction, where sulfur compounds like \(\text{SO}_2\) are converted back into hydrogen sulfide (\(\text{H}_2\text{S}\)) for easier removal. The resulting \(\text{H}_2\text{S}\) is then absorbed using a selective amine solution, which strips the pollutant from the gas stream and allows it to be recycled back to the main process unit for recovery. Alternatively, for tail gas with a high concentration of combustible pollutants, thermal oxidation or incineration is used. This process destroys the harmful compounds by combusting them at high temperatures, converting them into less harmful substances before the final release.