If it were possible to have perfect combustion, CO2 would be maximised and O2 would be at, or close to, zero in the flue gas stream. Since perfect combustion is not practically possible due, in part, to incomplete mixing of the fuel and air, most combustion equipment is set up to have a small percentage of excess oxygen present. The lower the temperature for a given O2 or CO2 value the higher is the combustion efficiency. This is because less heat is carried up the stack by the combustion gases.
As can be seen, the optimal combustion mixture is not displayed as a single point, but as a band of possibilities. The optimal combustion will depend on a number of factors and gives the possibility of adjusting for a reducing or oxidising gas, boht of which can be necessary in an industrial process. the maximum allowable CO value is also an important factor. This may not be exceeded at any time and may limit the adjustment of the burner for maximum efficiency. This should not be the case, but may occur on some older equipment, particularly when solid fuel is involved.
Smoke is the usual indicator of incomplete combustion in oil burners. In addition to indicating poor combustion, smoke can deposit soot on the heat exchangers, further reducing fuel efficiency as well. The real problem is the soot layer, but this ia seen by the public and operators as the smoke coming from the stack. The presence of smoke is a good indication that sooting will occur, hence the test for smoke is often referred to as a soot test.
Soot or smoke is one of the factors that will cause complaints from the general population. Carbon monoxide is invisible, but smoke can be seen from a long way away. Often it looks worse than it is due to the inclusion of water vapour in the stack gases. This will condense shortly after the exit from the stack and add to the smoke that is visible. Smoke is thus one of the factors that should be tackled also for good public relations and not just for the damage soot will do to the burner system. Soot can generally be removed fairly easily, but will require that the burner is switched off to do so. Some types of fuel will produce sooting to a certain degree regardless of all attempts to optimise the combustion, but this should be obvious and easily removed at the standard maintenance intervals. Burners that use a high level of excess air will naturally produce less obvious smoke than units operating in the most efficient zone. This may be one way of improving public relations, but his can probably be done at a lower cost in other ways.