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marengnr t1_j1e5fb4 wrote

Used to run oil fired boilers. CO was 0%. It was monitored, but if there was CO, that meant insufficient O2, which meant incomplete combustion, which meant $$$ going up the stack rather than making steam.

We didn't monitor NOx.

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genericTerry t1_j1fnrru wrote

Excess oxygen is also $$$ going up the stack as your heating up air as well as the combustion products.

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paulHarkonen t1_j1hxkvz wrote

Not to anywhere close to the same degree. Excess fuel consumption is way more costly than heating up a little bit of extra air.

Obviously you want to run as close to stoichiometric as possible, but since we are only human, running a bit lean is less expensive than running a bit rich (all other factors being equal).

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PerspectivePure2169 t1_j1eiotd wrote

The answer is going to depend tremendously on the fuel used, the conditions of burn, and any stack scrubbing or recapture equipment present.

Internal combustion engines have a starting advantage in that they tend to burn "cleaner" fuels - especially in the past. But external combustion systems have the advantage of larger plants and (generally) more space and attention to efficiency and emissions.

Looking at coal alone, there's a lot of variation among coal fields, as well as categorical differences between soft brown coal and anthracite.

But in any case, a cleaner starting fuel gives less to mitigate at the stack.

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magungo t1_j1etk9b wrote

The increased Nitrogen oxides in internal combustion is due to the compression phase of these motors. Given enough heat and pressure the Nitrogen from the atmosphere can be involved in more chemical reactions. Usually Nitrogen doesn't want to react much but we are forcing it to. This is why Diesel engines with higher compression ratios produce more Nitrogen oxides. As for CO that is more to do with incomplete combustion and is getting less and less with better engines running lean fuel mixtures and more computer control. In theory the Catalytic converter should be reacting most of the CO into CO2 but it's not going to be 100%.

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yurikovski t1_j1ez3ef wrote

The authoritative source on this is probably the US EPA AP-42 list of emission factors.

You could compare natural gas combustion in external vs internal. These are in terms of pounds of pollutant per mmbtu of fuel input.

In an uncontrolled natural gas turbine, you get 0.32lb/mmbtu NOx and 0.082lb/mmbtu CO. In a 4-stroke internal combustion engine, it's 4.08lb/mmbtu NOx and 0.317lb/mmbtu CO.

In a large boiler, it's 0.19 lb/mmbtu NOx and 0.082lb/mmbtu CO.

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