Air quality permits help minimize, reduce or prevent emissions as much as possible by placing requirements on how things are done. Permits can specify the quantity, type, or quality of fuel or other substance used in a process. For example, a permit might specify the maximum percent of sulfur that can exist in the coal to reduce sulfur dioxide emissions. A permit may specify the quantity of volatile chemicals in paint, solvent, adhesive or other product used in large quantity during manufacturing. Permits can also help reduce the impact of emitted pollutants on local air by specifying smokestack height and other factors.
Engineers can also set combustion specifications to minimize emissions.
For example, to help reduce nitrogen oxide formation, the combustion conditions in the furnace can be altered. The flame temperature can be lowered or raised, the amount of time air remains in the combustion chamber can be altered, or the mixing rate of fuel and air can be changed. These options are often reviewed, studied and best choices made depending upon cost, plant design and many other variables.