Composition of exhaust gases from humans and from fossil fuels

It occurred to me when carrying out some combustion calculations that what humans breathe out is pretty close to the flue gas from a gas-fired, gas turbine combined cycle plant.

In a gas turbine combustion chamber, fuel is burned typically at an excess air level of about 200% (the amount of oxygen available in the combustion air compared to that which is needed for complete oxidation of the fuel). This means that about one third of the oxygen available is used and converted to carbon dioxide and water while about 2/3ds just passes through (i.e of the 21% oxygen in air, about 6-7% is “consumed” and about 14 -15% passes through unused). In coal-fired plants the excess air levels are usually only about 25% which leads to about 15 -16% of the incoming 21% oxygen being consumed with about 5% passing through. The amount of oxygen actually consumed depends on the fuel composition and the oxygen demands of the elements which are oxidised during the combustion process. Carbon, hydrogen and sulphur (giving CO2, H2O and SO2) are the main oxygen consumers. All the other constituents of air pass through – heated up of course – but otherwise unchanged. Minute quantities of the fuel- nitrogen and the nitrogen in the incoming air can – depending upon the combustion temperature – be “fixed” to create the nitrogen oxides – nitrous oxide (N2O) and nitrogen dioxide (N2O). The higher the combustion temperature the greater the “fixing”. Too low a combustion temperature – for example with very wet fuels and bio-mass – can give “incomplete combustion” with some carbon monoxide (CO) and even some dioxins and hydrocarbons with a particularly poor combustion process. Internal combustion petrol engines essentially run at stoichiometric conditions (zero excess air) and there is no oxygen in the exhaust. However combustion is never quite complete and around 1% carbon monoxide is usually present (which is why suicide by exhaust fumes becomes possible). Diesel engines on the other hand have 10% oxygen in the exhaust when idling and this reduces to 1 or 2% when fully loaded.

All fuels essentially contain carbon and hydrogen as the main energy releasing elements when oxidised. Most industrial combustion processes happen fast and speed of combustion – which is desirable for complete combustion – has to be tempered by the need to keep temperatures at levels which can be handled by the materials used. The human use of the same elements of carbon and hydrogen for the release of energy however is by a relatively slow oxidation processes. Not all the water produced leaves the human body with our expelled breath since some part of it leaves in liquid form with urine. But from the composition of the waste gas we breathe out it seems that the carbon/hydrogen ratios in our food intake cannot be so very different to the natural gas burned in gas turbines (and not very surprising considering that plant-life is the ultimate source of both).

exhaust gas compositions


Since human exhaust gases emit the same concentration of carbon dioxide as gas turbine, combined cycle power plant perhaps we should penalise every human as well?



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