Water vapour
Combustibles losses
CO emissions NOx emissions Fuel usage
Hydrogen (H ) Nitrogen
Carbon monoxide (CO)
Typical O only operating set point
Combustion ine ciencies due to unburned fuel (combustibles)
Excess fuel
Useful heat transfer reduced
No excess O in ue gas
Air
300 ppm
Extremely high combustibles
200 ppm
CH + 1.6O + N Δ 0.6CO + 1.6H O + 0.4CO + 0.4H + N 15,500 BTU/Lb. = 35% less e cient (HEAT LOSS)
100 ppm
0%
1%
2%
3%
4%
Excess O
Figure 2 Optimum combustion set point
Figure 3 Air-to-fuel ratio reaction
When measuring moisture of other gas mixtures, only quartz crystal microbalance (QCM) non-equilibrium analys- ers have an internal calibration standard known as a moisture generator. These generators make use of the fact that most materials have a fixed specific permeation rate. The most common devices use a Teflon tube as the permeation device. The permeation rate will not change (no ageing effects) as long as the temperature and the flow to the device are constant. This permeation can work from the outside (with the tube in a water bath) or inside (with the tube filled with water). For accuracy of the device, flow through and temper - ature stability are essential. Undetected higher moisture concentration can cause pipeline/process equipment corrosion, line plugging due to freezing (ice formation), and lower process yield and/or lower product qualities. In any of these cases, the cost of such an event can be quite high and, in the case of corrosion, cause irreversible damage. Process heater optimisation One of the most common operating units in a refinery or any chemical processing plant is the process heater, used to force chemical reactions, combust products, or generate energy or steam. When deciding what to optimise on such a device, in today’s world we need to ensure that we optimise energy consumption as much as possible. This is typically achieved by controlling the combustion set point. Two control param - eters are important: one is the oxygen (O₂), and the other is the carbon monoxide (CO) concentration. Too much O₂ will result in heat losses and, therefore, less efficiency. If O2 is too low, the CO can be too high, which translates into ‘unburned’ fuel – a direct loss of energy. Figure 2 illustrates this function. A CO measurement is sufficient to optimise combustion, but this does not take into consideration the safety aspect of any combustion process. In any combustion process, there could be ‘unburned’ combustibles left, which may potentially create an explosive mixture. No combustibles displayed as CO equivalent (COe) are detected by any direct CO sensor systems. Instead, a catalytic sensor is required for this kind of measurement. Figure 3 shows an extreme example, but it is unlikely. Measuring COe is becoming more of a must when dis- cussing H₂ fired heaters. It is, therefore, important to note
that CO measurement is not equal to COe measurement. A final point for consideration involves a case of ‘flame lost’, which is a sudden loss of flame, meaning ongoing fuel sup - ply to the combustion chamber. An explosion can occur if undetected. This can be simply detected with one more sen - sor to measure CH₄. Instruments for clean energy projects Carbon reduction clean energy processes are increasing with the need for carbon capture. The most common projects involve H₂-generating units. The three key technologies for producing H₂ are: • SMR: Steam methane reforming units • ATR: Auto thermal reforming • SGP/POx: Shell gas partial oxidation/partial oxidation. In order to call hydrogen (H2) blue, carbon capture must be implemented. There are several components to be measured to monitor process reliability and maintain close process control. The requirement here is for multicomponent/multist - ream and flexible instrumentation. The right combination needs to be defined. As with all previously discussed points, the ‘golden rule’ of sample system engineering applies: know your process conditions; invoice the right people; simplify the system; and select the right equipment. SRU process optimisation The following tail gas treating unit (TGTU) discussion is an amine-based TGTU within the sulphur recovery unit (SRU). The purpose of the TGTU is to convert all remaining sulphur components carried over from the modified Claus unit into H₂S. This happens in the catalytic section of the TGTU pro - cess. The reduction reactor utilises a cobalt-molybdenum catalyst (also mentioned as CoMo bed). The CoMo catalysed reactions are shown as follows: SO₂ + 3H₂ g H₂S + 2H₂O S + H₂ g H₂S
H₂O + CO g H₂ + CO₂ COS + H₂ g CO₂ + H₂S CS₂ + 2H₂O g CO₂ + 2H₂S
The first step of the reaction does require H₂, which can be present as a byproduct of the modified Claus reaction and also generated by an inline reduction burner or provided from an external source. Regardless of the H 2 source, to ensure
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PTQ Q1 2025
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