THE COMTHERM ON-LINE TECHNICAL MANUAL

COMBUSTION QUALITY

The hot gases produced by the combustion process are referred to as the PRODUCTS OF COMBUSTION (sometimes referred to as ‘flue gases’). Examination of combustion reactions will show that these products of combustion consist mainly of carbon dioxide (CO₂), water vapour (H₂O) and nitrogen (N₂); excess oxygen (O₂) (when excess combustion air is used) will also be present. Other flue gas constituents may be present in very small but undesirable quantities; these include carbon monoxide (CO), aldehydes (HCOH, CH₃COH) and noxides (NO, NO₂)

Perfect Combustion is the complete oxidation of the combustible elements of a fuel with that quantity of air containing just the theoretical amount of oxygen required. Incomplete (imperfect) combustion normally occurs when a sufficient amount of air to completely oxidize all of the combustible elements in the fuel has not been supplied, with the result that some of the carbon in the fuel will be oxidized only to CO. As the quantity of air is decreased, the products of combustion will contain, among other products, increasing amounts of CO and decreasing amounts of CO₂.

Imperfect Combustion also includes those combustion reactions which are not complete despite the presence of sufficient Oxygen. For example, in systems in which CO is formed due to poor mixing, quenching of flame due to impingement on cold surfaces, etc. Complete Combustion is the complete oxidation of the combustible elements of a fuel whether it is secured with just the theoretical amount of oxygen required for perfect combustion or with some quantity of oxygen (or air) in excess of theoretical requirements.

The relationship between CO₂ and oxygen in the products of combustion of methane gas, is illustrated in Fig CT-10a.

CARBON DIOXIDE content is the main parameter used to give an indication of combustion efficiency (see section on combustion efficiency). Ideally in indirect fired applications e.g. water & steam boilers, the CO₂ measurement should be the theoretical maximum, however excess air is normally present to ensure complete combustion and minimise carbon monoxide (CO) production.

The ultimate CO₂ in flue gases is the maximum percentage which a given flue gas may contain under conditions of perfect combustion. This figure usually varies directly with the total H₂ content of the fuel and is about 12% for most natural gases on the dry basis of analysis. If a fuel is solely composed of carbon (C), the ultimate CO₂ would be 20.9%. When flue gases are analysed, the water vapour formed by the combustion of H₂ in the fuel is condensed out in the analyzing equipment and does not show up as one of the constituents in the final analysis; the remaining constituents, therefore, are always reported on a dry basis.

The graph Fig CT-10a can be used to provide CO₂ content data if only the O₂ content is measured (or vice versa). The curve shows the maximum theoretical carbon dioxide content in the products of combustion as 11.51% at stoichimetric conditions; above the stoichimetric air:gas ratio the CO₂ value decreases because of the dilution affect of the excess air and the oxygen content increases. Carbon dioxide content measurements are made on dry gas so the theoretical CO₂ contents indicated in most data tables or publications do not take into consideration the volume of water vapour present.

CARBON MONOXIDE presence in the products of combustion is normally the first indication of poor combustion. The formation of carbon monoxide is of particular interest to combustion engineers because of its poisonous affect on personnel (it attacks blood haemoglobin) and its measurement is the most common parameter used when measuring combustion quality.

The combustion graph Fig CT-10a shows that below the stoichimetric mixture the CO decreases as the CO₂ increases. The dotted CO line on the graph indicates that in fact unlike the theoretical case some excess air is required to reduce the CO to a minimum value. Some small carbon monoxide content is almost always present in products of combustion and the concentrations allowed are stipulated in various standards depending on type of burner application.
To have meaningful carbon monoxide standards we have to take into consideration the amount of fuel burnt; for this reason the measurement of carbon monoxide must take into consideration the carbon dioxide content of the products of combustion or translate the carbon monoxide measurement to stoichimetric conditions. It is common to translate the CO performance of a combustion system into a CO:CO₂ ratio.

The EN676 standard is the European standard for fully automatic packaged burners and although not applicable to process burners is the best and strictest guide available for CO requirements. This standard stipulates a maximum CO production of 100mg/kWh - this is a direct relationship between CO produced and fuel burnt; it can be translated to a value of 93 p.p.m. in dry airless products of combustion.

The EN525 standard which applies to direct gas fired air heaters where products of combustion are present in the main air supply being used to heat a factory space or any other occupied enclosure, allows a maximum of 10 p.p.m. of carbon monoxide (CO)

ALDEHYDES can be produced during the combustion process, these organic compounds are only produced in small quantities but however they can have unpleasant effects. Aldehydes are usually caused by some form of flame chilling.

The most common of the aldehydes to be formed are acetaldehyde (CH₃CHO)and sometimes formaldehyde (HCOH). The formation of these compounds is a problem as they can cause sore eyes, headaches etc. even in very small quantities. In the case of direct fired gas burner applications were products of combustion pass into working space then strict standards exist to limit production of aldehydes.

British standard BS5990 which applied to direct fired air heaters, stipulated a maximum aldehyde concentration of 0.4 p.p.m. in the discharge air from a heater. It should be noted however that BS5885 which applied to fully automatic systems and BS5991 which applied to indirect fired air heaters contain no requirements with regard to aldehyde concentrations. The European standard EN1020 which applies to indirect fired air heaters and EN676 which applies to fully automatic burners specify no aldehyde limits in products of combustion.

NOXIDES (oxides of nitrogen) have in recent years become a concern to the combustion engineer. Requirements for Low-Nox burners are on the increase, due to environmental legislation. Noxides are toxic and can be harmful both to our infrastructure and the environment and do cause health problems. Noxides can be far more destructive than the more publicised problem of C0₂.

Noxides are formed by natural phenomena such as lightning and are also created within the combustion process by air passing through the hot flame; it is therefore reasonable to expect that noxide production is relative to high flame temperatures. Noxide production also depends on local application conditions such as humidity and pressure. The most common noxides formed are nitrogen dioxide (NO>sub>2) and nitric oxide (NO) and total Noxides (NO_X) is expressed as the total of both contents.

Within the combustion process Noxides can be produced in three ways. Firstly there is NO_x formed by the thermal process, (thermal NO_x), this accounts for approximately 90% of all NO_x produced by the combustion of natural gas and about 50% of that produced by oil burning. NO_x originating from the fuel combustion reactions itself accounts for around 5-10% of total Nox in natural gas and 40% in the case of oil).

Within the burner industry attempts have been made to reduce noxide emissions from burners; these attempts have been concentrated on the lowering of flame temperatures by either creating radiant flames or by arranging partial recirculation of products of combustion through the flame.

The BS5990 standard that applied to direct gas fired air heaters stipulated a maximum noxide concentration of 6 p.p.m. in the discharge air. The standard stated a maximum for nitric oxide (NO) of 5 p.p.m. and for nitrogen dioxide (NO₂) 1 p.p.m. EN676 which is the standard for fully automatic burners, specifies maximum noxide limits in the case of 2nd family gases (natural gases) of 170mg/kWh and in the case of 3rd family gases (L.P.G.) 230mg/kWh.

No_x levels can be recorded on either a 'wet' or 'dry’ basis; this can cause some confusion when Nox ppm levels are quoted as the most accurate Nox readings are 'wet' samples yet most portable type NO_x analysers do filter and partially 'dry' the flue gases through a condensate trap arrangement.
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NO_x levels are not in themselves an accurate guide to combustion performance, they must always be referred to in terms of the oxygen content. The methods used in NO_x measurement and claimed performances have to be carefully considered. Clearly by over airing a flame it is possible to reduce the NO_x level for example from say 40 ppm at a O₂ content of 4.2% to 28 ppm with a O₂ content of 7.1%.

Oil firing presents additional NO_x problems over gas firing as the nitrogen content of the fuel is much higher and so has an obvious effect on the actual NO_x levels. Some standards refer specifically to light oil with a nitrogen content of 140 Mg/Kg of fuel; it is expected that for every mg. of nitrogen exceeding this figure, the N0_x would increase by approximately 0.2 mg/M³.

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