Optimizing NOx emission reductions in industrial natural gas combustion processes

Effective complementary techniques

NOx emission reduction techniques are grouped into two main categories: those that act at the time of combustion and can modify combustion (combustion modification techniques) and those that occur after combustion (post-combustion techniques). After an overview of NOx formation mechanisms and a brief comment on these two types of control techniques, we will look at reduction techniques in more detail.

Formation of NOx in natural gas combustion processes

NOx emissions are composed of nitrogen oxide (about 95%) and nitrogen dioxide (about 5%). During natural gas combustion, three separate mechanisms are involved in NOx formation:

Oxidation of nitrogen chemically bound to the fuel (fuel NOx);
Reaction of hydrocarbons and nitrogen, which produces oxidation (instantaneous NOx); and
Oxidation of nitrogen in the combustion air that occurs at high temperature (thermal NOx).

Of these three mechanisms, thermal NOx contributes the most to NOx emissions, as shown in the table below.

Table 1: NOx formation by energy source

Energy source	Thermal NOx	Instantaneous NOx	Fuel Nox	Total
Natural gas	85%	15%	0%	100%
Heavy fuel oil	30%	10%	60%	100%

The formation of thermal NOx mostly depends on temperature: below 1300°C, there is virtually no NOx formation, while above 1550°C, NOx formation is rapid and continuous.
Instantaneous NOx, for its part, is primarily produced during combustion in flames loaded with hydrocarbon fuel. NOx emissions for natural gas are expressed in parts per million (ppm) at 3% O².

Combustion modification techniques

Combustion modification techniques work by limiting the amount of NOx produced at the source (primary measurements). To modify combustion, certain changes need to be made to the burner and combustion chamber design and/or modify the rapid combustion technique. These techniques generally require a lesser investment than post-combustion techniques and have lower operating costs. They reduce NOx by 50-60%.

The most common of these techniques are staged combustion and temperature reduction :

Staged combustion consists in changing the air/fuel ratios in the flame. For example, a fuel-rich flame is sought, with a contour with more air in order to create conditions less conducive to NOx formation. Ultimately, the ratios rebalance, resulting in complete combustion with lower NOx production.
Temperature reduction involves introducing some of the flue gases into the combustion air, which will lower flame temperature by reducing the available oxygen.

Post-combustion techniques

Once these techniques are implemented, NOx can be further reduced using post-combustion, which reduces emissions as they form in the combustion chamber. These techniques convert NOx into harmless products through chemical treatment (secondary measures).

Selective catalytic reduction

Selective catalytic reduction (SCR) is currently the most successful post-combustion technique with a NOx reduction of 80-90%. SCR requires the use of a reactor where the catalyst (set of active solid particles) is stored. Ammonia (NH₃) is injected into the flue gas prior to being introduced into the reactor. The role of the catalyst, or more specifically the catalytic bed, is to reduce the temperature of the window where the chemical reaction occurs between the ammonia injected into the flue gas and the NOx. Minimum operating temperatures range from 200°C to 450°C, while maximum operating temperatures range from 400°C to 550°C.

Catalysts

Three types of catalysts are currently used in SCR systems:

The first, which is metal oxide-based, uses titanium-vanadium oxides that are incorporated into a metal or ceramic substrate. These oxides can also be contained in a homogeneous ceramic substrate. This is the most common type of catalyst in SCR systems.
Increasingly more widespread, the second type of SCR catalyst is the zeolite molecular sieve catalyst. NOx and ammonia are adsorbed and react in the porous structure of the catalyst, which allows operating temperatures to exceed those of the metal oxide-based catalyst. In addition, there is no fouling or contamination from fuel residues.
The third type of catalyst is precious metal-based and converts residual ammonia into nitrous oxides, which are not considered to be NOx based on the federal definition. Carbon monoxide is oxidized simultaneously. Operating temperatures are significantly lower than those of metal oxide-based catalysts, making this system more susceptible to sulphide contamination.

The following table compares the NOx emission reduction rates and associated operating costs for three different techniques.

Table 2: NOx reduction rate in relation to operating costs

Techniques	Selective catalytic reduction	Gas recirculation
NOx reduction rate (% vol.)	80 to 90% and above	60 to 70%
Cost ($K/tonne NOx)	$7 to $16K/t	$2 to $8K/t