Reducing NOx from Fired Heaters and Boilers
Look at NOx time line and find out where you stand with respect to NOx compliance.
Introduction
Since 1970, EPA has been tracking six principal pollutants- carbon monoxide, lead, nitrogen oxides, particulate matter, sulfur oxides, and volatile organic compounds. All have decreased significantly since the passage of the clean air act in 1970 except for nitrogen oxides. Reducing NOx emissions is a crucial component of EPA’s strategy for cleaner air. Nitrogen oxides are formed when fuel is burnt at high temperatures, as in a combustion process. The primary sources on NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuel. In the last three months, Houston has taken over Los Angeles as the smoggiest city in USA.
In the last 25 years, a number of technologies have surfaced and have been implemented in process heaters and industrial boilers. In this series, we are going to take stock of the state of art NOx reduction methods and what can be expected in the next decade as we continue our battle for NOx reduction..
What is NOx
What is NOx? Nitrogen oxides or NOx is the generic term for a group of highly reactive gases all of which contain nitrogen and oxygen in various proportions. Many nitrogen oxides are colorless and odorless. . Two of the most common oxides of nitrogen are: NO - Nitric Oxide and NO2 - Nitrogen Dioxide. In stationary source, combustion approximately 90% of NOx formed is NO. After NO leaves, a stack, in the presence of sunlight, ozone, and VOCs, it becomes NO2, which (in extreme cases) appears as a reddish-brown plume. It can cause photochemical smog and/or acid rain
How is NOx formed?
There are several ways that NOx is formed in a furnace. The thermal fixation of atmospheric nitrogen and oxygen in the combustion air produces “thermal NOx; while the conversion of the chemically bound nitrogen in the fuel produces “fuel NOx”.
For natural gas and light-distillate-oil firing, nearly all NOx emissions result from thermal fixation.
The formation rate of thermal NOx is dependent on the reaction tempera­ture, the local stoichiometric, and the residence time. The fuel NOx formation mechanism is more complex depending upon fuel pyrolysis and subsequent reaction between many intermediate nitrogenous species and the oxidant species.
Thermal NOx is most readily influenced by the combustion system.
“PROMPT NOx is produced by the breakdown of CH portions of methane and other hydrocarbons in the fuel and their subsequent combination with nitrogen in the air.
The rates for formation of both thermal NOx and fuel NOx are kinetically or limited, with the amount of NOx formed being much less than the equilibrium value. The rate of formation of NOx is dominated by combustion conditions and can be suppressed by modifying the combustion process. Both thermal and fuel NOx are promoted by rapid mixing of oxygen with the fuel Thermal NOx is greatly increased by long residence time at high temperature.
In the next part of this article, we are going to review NOx reduction through burner in fired heaters and boilers.

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