Decarbonisation Technology May 2022 Issue

Role of combustion optimisation As its second role, flue gas analysis also offers the ability to optimise the excess oxygen setpoint using a ‘combustibles’ measurement. For context, combustibles are generated as a byproduct of incomplete combustion at the burner. Under perfect conditions, hydrocarbon fuels react to form carbon dioxide (CO₂) and water (H₂O). However, combustion is never perfect in practice because of poor air/fuel mixing at the burner, changing load conditions, malfunctioning burners, and variable fuels. As a result, a small amount of unburned combustibles is always generated, usually ppm levels of carbon monoxide (CO) and hydrogen (H₂). A combustibles detector uses catalytic elements to measure CO and H₂ together in a single measurement. As shown in Figure 1 , the combustibles detector uses a catalytic active element and a reference element to provide this single, combined, non-speciated ‘umbrella’ measurement. In the case of high hydrogen fuels, these catalytic combustibles detectors have higher sensitivity to monitor for unburnt hydrogen. However, they can also monitor for CO if hydrocarbons are present in the fuel source. That said, the combustibles measurement can be used to monitor the health of the combustion process, as combustibles inform the operator of how much ‘incomplete combustion’ is present. As the excess oxygen increases, fewer combustibles are formed. However, if insufficient excess oxygen is present, the combustibles increase dramatically. In extreme cases, where the burner is starved of enough oxygen, the combustibles level hits a point of ‘breakthrough’ and increases exponentially, as shown in Figure 2, creating an unsafe condition. That said, operators can use the combustibles measurement (in conjunction with the excess oxygen measurement) to reduce their carbon emissions and fuel consumption – ultimately ensuring safety while also optimising their combustion process. As noted earlier, operators use the excess oxygen measurement as a setpoint for their burners, but excess oxygen alone does not tell the full story in the flue gas. Too much excess oxygen, and it reduces fuel efficiency. Lower excess air levels mean there is less air and flue gas to heat, and thus less heat

Reference element

Catalytic sensor

Combustibles sensor housing

Catalytic element

Flue gas ow

air may be needed at the burner, although the addition of a combustibles reading would give a fuller picture of the combustion process (which will be described in the next section). That said, insufficient oxygen at the burner causes incomplete combustion, an obvious waste of fuel, and a potential safety hazard. Through flue gas analysis, operators can ensure that the burner has enough air to maintain a safe and stable flame, and this is especially true when using high hydrogen fuels. Figure 1 An example combustibles detector, which uses catalytic elements to monitor for ppm levels of incomplete combustion, including H 2 and CO, in a single, combined ‘combustibles’ measurement

Excess air losses

Combustibles losses

Eciency combined losses

O & combustibles Optimum Control Point

O only operating set point

300 ppm 200 ppm 100 ppm

0% 1% 2% 3% 4%

Excess O

Figure 2 Measurement of excess oxygen and combustibles enables optimised combustion and lower fuel consumption and stack emissions