A Focus on Combustion Analysis

Technical Specifications at EOGB Energy Products Ltd take an in-depth look at combustion analysis and how the data produced can be used to ensure both optimum safety and performance…

With the introduction of increasingly stringent legislation to reduce the amount of post combustion gases produced and achieve optimum performance from modern high efficiency appliances, it has become necessary to rely more upon the use of portable combustion analysers than ever before.

Although combustion analysis equipment has been used for many years, it has previously only been for specialist and non-domestic applications. Combustion analysers were not commonly used in the domestic gas market until the introduction of the high efficiency ‘A’ rated condensing appliances.

As it is not always possible to establish the quality of combustion by the physical appearance of a flame alone, and coupled with the necessity to commission certain types of appliance within very tight parameters, it has become impossible to install, commission, service or repair certain appliances without the use of an electronic combustion gas analyser (ECGA).

Interpreting data

To understand the information that the ECGA is relaying back to us, it is first necessary to understand the basic combustion equation for any hydrocarbon fuel (see figure 1.1).

In practice it is normal to introduce more combustion air than that which is required to achieve stoichiometric combustion, so the equation must be interpreted as figure 1.1 if one assumes complete combustion.

C_xH_y + _aO₂(O₂ + 3.76 N₂) è _xCO₂ + _yH₂O + _aO₂ + _aN₂

                                                                   Figure 1.1

It can be observed that constituents present in the products are the variables being measured/analysed by the ECGA and the subsequent resultant information is displayed for the engineer to interpret.

The readings provided by the ECGA allow the engineer to adjust the fuel air mixture within the appliance to achieve its optimum level of efficiency and safety. This requires a firm grasp of the combustion process and an ability to interpret the information being displayed on the ECGA.

Focusing on safety

Current documentation does not necessarily provide specific details of the combustion process but instead focuses upon a set CO/CO₂ ratio to enable engineers to determine whether the appliance is operating safely. It is fair to say the CO/CO2 ratio (calculated) is an indication of the combustion safety rather than combustion efficiency.

BS 7967 outlines CO/CO₂ action levels (appliance dependent) which determine the level of service a specific appliance requires and an action level. Safety should always be the first consideration, but with correct use of the ECGA it is possible to achieve both optimum and safe performance if the individual variables on the ECGA screen are given closer attention and not all focus is given to the CO/CO₂ ratio alone.

CO/CO2 ratio

Figure 1.1 shows that a specific fuel will yield a fixed percentage of CO₂ and H₂O when a prescribed volume of fuel is completely burned in theoretical (stoichiometric) air. Any deviation in the quantity of fuel or air from this amount will alter the products and have a resultant effect on combustion safety and efficiency.

CH₄ + 2O₂ (O₂ + 3.76N₂)  CO₂ + 2H₂O + 7.52N₂

From the above equation the complete combustion of methane will yield 1 mol of CO_2, 2 mol’s of H₂O and 7.52 mol’s of N_2. Overall a total of 10.52 mol’s of combustion products exist, assuming the water remains in vapour state (measured wet) and the heat of vapourisation cannot be utilised by the appliance (non condensing appliance).

A simple calculation can be performed to determine that the complete combustion of 1 mol of methane under stoichiometric conditions will yield (wet):

CO₂ = 9.5% CO₂ by volume

The aforementioned CO/CO₂ ratio is derived from the measured quantity of CO and normally, but not exclusively, calculated CO₂.

Example 1.1

6.5% CO₂ × 10,000 = 65000ppm

15ppm CO measured

The ratio is obtained by the applying the following calculation:

Giving a CO/CO₂ ratio of 0.0002

Although the ratio indicated above is within all the tolerances outlined in BS 7967 it has no bearing upon the overall combustion efficiency of the appliance as it is in effect a dimensionless number.

With the correct understanding of the ECGA and to the type of flame being analysed, efficiency can be improved whilst still ensuring complete combustion.

Correct readings

Consideration must also be given to where the combustion sample is being taken, as the combustion products could be diluted down due to comburent air being drawn in from a draught diverter. For example, it is not always possible to extract a combustion sample from the primary flue due to appliance design.

If we take a reading from such a location it is not uncommon to experience CO₂ readings as low as 2-3% as the fresh dilution air is diluting the combustion products and it is at this point where the CO/CO₂ ratio becomes greatly significant. For example, a reading of 200ppm CO and 2.9% CO₂ have been extracted from a sample point downstream of the draught diverter.

At first glance the quantity of CO appears to be within the action level of 350ppm stated in BS 7967, but if the same calculation is applied to derive a CO/CO₂ ratio we can find it throws up some interesting facts.

If we take the reduced calculated CO₂ value and refer it back to a theoretical reading of 11.9% the above equation can be manipulated to provide the following results:

The above equation identifies that the relatively low concentration of CO is potentially increased to a significant 820.7ppm depending upon the level of dilution taking place.

The potential exists for significant CO levels to be formed. By taking the CO₂ up to its maximum potential volume the CO can exceed the allowed limit set out in BS 7967 when it is not diluted. The calculation also identifies that indeed at least some of the combustion is not complete, having not only a deleterious effect upon combustion efficiency but also upon safety.

This can appear to be a contradiction if one adheres to the relevant standard and focuses on the CO/CO₂ ratio alone. With an enhanced insight into combustion analysis it becomes clear that the ability to interpret not just the CO/CO₂ ratio but in fact all of the information the ECGA is providing us, is essential.

It should also be understood that by having a greater understanding of how different types of appliances utilise different combustion methods, the heating/gas engineer can further rely upon their ECGA as an invaluable tool in ensuring both appliance safety and increased appliance efficiency.