Posts Tagged ‘NO’

Another reason to stop using gas

March 6, 2022

Friends, many people are considering reducing, or stopping entirely, their use of natural gas for heating and cooking. Perhaps you are one of these people.

It may be that your motivation is because burning this gas is altering the climate of our planet.

Or may be that your motivation is because buying the gas supports murderous and megalomaniacal regimes across the plant.

But perhaps these motivation aren’t quite enough. If so, then please consider this:

  • Cooking with gas is poisoning you and your family

Yes, cooking with gas emits nitrogen oxide (NO) and nitric oxide (NO2) into your kitchen. Collectively these gases are known NOx.

When NOx reaches the membranes of your skin or nose, it quickly forms nitric acid, which irritates the membranes and can cause asthma and sensitise people to other allergens.

If you are concerned about air pollution in cities, then before worrying about vehicle emissions, you should probably first focus your attention on your own home where NOx levels are likely to be very much higher.

Let me explain

Air consists of very roughly 80% nitrogen (N2) and 20% oxygen (O2).

When burning natural gas, methane (CH4), in air, the majority of the combustion products (water (H2O) and carbon dioxide (CO2)) arise from reactions between methane and oxygen.

The nitrogen molecules – despite making up the bulk of the air – are relatively inert. But they are not completely inert.

At the high temperatures – approaching 2000 °C – of a methane flame, the nitrogen and oxygen molecules dissociate into atomic nitrogen and oxygen and in this state they react to form oxides of nitrogen, primarily NO.

This NO then converts to NO2 over a time frame that depends on what else is in the atmosphere. Thus even when the amount of NOx is constant, the fractions of NO and NO2 are likely to change over time.

When methane combustion takes place in a boiler, none of the combustion products enter your home.

But when you cook with gas, the combustion products are all vented directly into your home. Including the NO and NO2 i.e indoor NOx pollution.

Is this really a problem?

I don’t know for sure, but I suspect it must be.

Whereas professional kitchens frequently have strong extraction over open burners and ovens, domestic kitchens often do not. And where extraction is present, it is often not used, and when it is used, it only covers burners and not ovens.

Concentrations of NOx are difficult to measure for several reasons.

Firstly a meter to measure NO2 costs thousands of pounds versus a hundred pounds or so for a CO2 meter, and so there are very few reported measurements in kitchens.

And secondly, the ratio of NO to NO2 is generally not well-known in any particular circumstance.

Consequently using measurements of NO2 to estimate NOx will always give an underestimate of the NOx level.

One measurement in kitchens is in this article.  It shows measurements of NO2 during an evening of cooking in one US household. I have reproduced the figure below.

Click on figure for a larger version. While cooking with gas in this US household, NO2 levels rose to almost 300 ppb. This figure is modified from the linked article.

In the UK exposure limits for NO2 are an annual average exposure to 40 μg/m^3 with less than 18 exposures per year to peaks above 200 μg/m^3 averaged over 1 hour.

So it looks like the occupants of this household are being exposed to very high levels of NO2. But the  NO levels close to the cooker are likely to be even higher.

My Measurements and Calculations

I wondered if the measurements above were plausible. The peak did not have the shape I would have expected: it seems to fall very rapidly suggesting there was strong airflow through the house.

Unfortunately, I can’t measure NO or NO2 directly but I routinely monitor CO2 in the central part of the house, well away from the oven and hob. Nonetheless I regularly see the CO2 levels rise to over 1000 ppm during cooking. For this article I also took measurements with the detector at roughly head height next to the hob.

Click on figure for a larger version. The location of the CO2 meter relative to the hob for the  measurements in red in the graph below.

The graph below shows the CO2 data.

  • For the detector near the hob, the burner was on for 15 minutes and the CO2 levels rose immediately.
  • For the detector in the neighbouring room, the burner was on for 17 minutes and there was a delay of many minutes before CO2 levels began to rise.

Click on figure for a larger version. The rise in carbon dioxide concentration above background resulting from a single gas burner on teh hob. The measurements in black were measured several metres away in a different room. The measurements in red were measure at head height above the hob..

What is clear from both these measurements is that CO2 concentrations of at least 500 ppm above background are likely to be commonplace in all the rooms in homes which use gas hobs, ovens or grills.

I wondered if the ratio of production of NO to CO2 might occur at a fixed ratio. If so, that would allow me to use measurements of CO2 concentration to estimate likely levels of NO.

I wasn’t quite sure how to do this but an old friend suggested using the free and excellent GasEq software to calculate the likely combustion products and their relative concentrations.

Using the methane combustion in air example, I calculated the ratio of the NO in the exhaust gases to CO2. Then from measuring the CO2 rise due to combustion, I could estimate the NO concentration in the house.

Click on figure for a larger version. Logarithmic graph showing estimates of the NO concentration in parts per billion (ppm) assuming 500 ppm CO2 concentration from combustion and 400 ppm CO2 background concentration i.e. a measured CO2 concentration of around 900 ppm. See text for further details.

At first I was shocked. The calculation suggested that NO levels of several thousand ppm were likely. But this was based on two assumptions: that the gas flame was adiabatic and stoichiometric. What wonderful words.

  • Adiabatic means that no heat is lost from the flame and so the products would be at their maximum possible temperature, approximately 2225 K. However in a domestic gas burner, heat will be lost to both the burner itself, and saucepans which typically only reach 250 °C. So I repeated the calculation for lower temperatures.
  • Stoichiometric means that exactly the right amount of oxygen was mixed with the methane so that all of the methane and oxygen reacted.
    • If the gas mixture has excess methane (a so-called fuel-rich mixture) then less oxygen will be available to react with the nitrogen, and NO production will be reduced.
    • Similarly If the gas mixture does not quite enough methane (a so-called fuel-lean mixture) then some un-reacted oxygen will be available to react with the nitrogen, and NO production will be increased.

So I repeated calculations for a range of stoichimetries (±5% and ±10% from ideal) and a range of temperatures, extending down to more than 200 °C below the adiabatic flame temperature.

My conclusion from this calculation is that even with very conservative assumptions, when CO2 levels from combustion rise 500 ppm above background, the levels of NO in the air is likely to be several hundred ppb. Eventually some fraction of this NO will convert to NO2 and yield NO2 levels well-above safe exposure levels.

Of course without direct measurements, I don’t know this for sure, but I am surprised that this issue is not discussed more.

Summary

My conclusion is simple. Based on measurements of CO2 concentration in my own home, and calculations of the likely ratio of NO to CO2, I think that NOx exposure in UK households with open gas hobs, ovens, and grills is likely to routinely exceed exposure guidelines.

For people standing over a hob, or people routinely working in a domestic kitchen, exposure levels could easily be dramatically higher.

If anyone has problems with asthma or is concerned about their own – or their children’s exposure to air pollution – then it is likely that the best thing people can do is to stop using gas for cooking, and to instead use microwaves, electric ovens and induction hobs.

This archaic ‘burning’ technology is funding Putin’s war machine, changing the Earth’s climate. AND polluting my home!

Personally, I just can’t wait to get rid of this gas hob as soon as possible.


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