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Why the knock on NOx?

We’re kicking off a new series of posts on the THM blog this week, all of them focused on emissions.  Depending on the area of the country/world that you live in, emission regulations can be wildly different.  In the U.S. emissions have been a topic of discussion in the oil and gas industry for many years.  But most of the discussion has historically been focused on the downstream portion of the business – refineries and petrochemical plants.  Less attention has been paid (relatively) to the processing and gathering businesses, until recently.  When looking at combustion equipment there are several common emissions that should be evaluated:

  • NOx – nitrous oxides
  • CO – carbon monoxide
  • UHC – unburned hydrocarbons
  • VOC – volatile organic compounds
  • PM – particulate matter
  • SOx – sulfur oxides

We’ll look at each of these emissions throughout the series, discussing what the emission is, why it is harmful, and what we can do to reduce the emission.  We will start the series talking about NOx (pronounced like ‘box’).

Firstly, what is NOx?  During the combustion process, Nitrogen and Oxygen molecules from the air and fuel combine to form NO and NO2, which we commonly combine and call NOx.  The main driver of NOx formation in the combustion process is the flame temperature.  If you can lower the flame temperature, you can typically lower the NOx emissions.

But why is NOx a bad thing?  NOx has been proven to play an active role in the formation of ozone.  Ozone high in the sky is good (helps protect us from the sun’s radiation), but ground level ozone is not so good for us.  High ground level concentrations of ozone usually occur in urban areas where lots of NOx and volatile organic compounds (VOCs) are released.  NOx, while invisible by itself, can create a visible smog when combined with VOCs and sunlight.  Reducing NOx and VOCs has a visible impact on smog.  I’ve been travelling to the LA area for the last 30 plus years and was involved in many furnace NOx reduction projects over that same time period.  I can personally attest that the skies have become cleaner. 

When the NOx rules for furnaces in California were first adopted (back in the 80’s) there was no viable burner technology to achieve it.  The combustion (burner) industry responded and produced NOx reducing technology that cut NOx emissions by half of what was available previously.  Larger heaters had to achieve 0.03 lb/MMBtu NOx emissions and the best staged fuel technology at the time could only do around 0.05 lb/MMBtu.  The industry called this Ultra Low NOx technology. 

The regulators came calling again in the late 90’s and mandated further reductions with refiners and had them implement comprehensive NOx reduction targets that necessitated new technology.  The old Ultra Low NOx burner was not going to cut it any longer.  The combustion industry responded and came up with burner technology that reduced the NOx again from a nominal 0.03 lb/MMBtu to 0.02 lb/MMBtu.  The industry had already used up all the obvious nomenclature - Low NOx burners, Ultra Low NOx burners, so they now coined the new stuff as “Next Generation” technology.  This technology came of age in the early 2000’s.  Since that time, there have been no significant progress in a “burner only” solution.  Options to go lower than 0.02 lb/MMBtu levels are external flue gas recirculation (FGR) and SCR’s (selective catalytic reduction).  The external flue gas option has been creeping the NOx down lower and lower and seems to be where the technology race (focus) has been.  Some suppliers can offer as low as 0.008 lb/MMBtu.  SCR’s can go down even further to 0.002 lb/MMBtu however, there is the hassle of using/needing ammonia to run this technology.

The “burner only” technology is the most budget and operator friendly option and does the trick about 90% of the time.  Using this technology requires no additional controls or operator techniques when compared to a conventional NOx burner.  Flames are little longer and it’s best if the heater has been designed around this technology from start, however, numerous retrofits have been accomplished over the last 15 years this technology has been available.

Our advice on buying Next Generation technology or FGR in combination with Next Generation is:

  1. Ask where a similar installation is in service and what challenges if any have been experienced
  2. Test anything that is new.  This is a fired test at the factory using the real burner and simulating the fuel, firing rate and furnace temperatures as closely as possible.
  3. Don’t buy stuff that takes lots of special controls, extra control loops etc.
  4. Only buy burners from suppliers that have full size test facilities and field service staff.

Still have questions on NOx?  Contact us and we can help.


We’ll continue our series on emissions next week when we are going to talk about CO emissions.  Make sure to check it out.