Author: Kenneth Vindum
Co-author: Peter Blinksbjerg
Release date: June 27, 2023
Many facilities around the world will be implementing Carbon Capture (CC) in the near future to reduce their CO2 emissions. It is crucial for emitters to implement the technology to meet global and national greenhouse gas reduction targets within the framework of the green transition and “Fit For 55”, the EU’s green transition plan.
Figure 1. Test and measurement setup for a waste-to-energy plant retrofitted with a Carbon Capture system
At Olicem, we suggest that plants continue to follow best practice by measuring the flue gas just before it leaves the stack. This will provide the most reliable results and be the cheapest method. However, this means that an additional dryO2 measurement must be taken and measurements taken after CC must be corrected for the capturedCO2 before they can be compared to emission limits using a correction factor. The approach has several advantages:
- Current legal requirements can still be applied (we don’t need to wait for the EU to issue revised emission limit values or new BREF documents).
- Concentration-based emission limits will remain unchanged regardless of whether the CC system is running or not.
- The regulatory requirements for the operation of the facility will remain unchanged.
- The principle of measuring last before the flue gas leaves the stack is maintained.
The overall problem statement
For simplicity, the main components of 100m3 of flue gas are illustrated in table 1 below. The example also illustrates the components of the flue gas before a carbon capture system.
Table 1. Illustration of the main components of 100m3 flue gas before the CO2 absorber (current scenario)
The oxygen concentration in dry flue gas is calculated as:
When the 100m3 passes through the CO2 absorber, approximately 90% of the CO2 will have been removed from the flue gas. This is illustrated in Table 2 below, where 10m3 of CO2 has been removed.
Table 2. Illustration of the main components of 90m3 flue gas after the CO2 absorber
The oxygen concentration in dry flue gas is calculated as:
Note that the volume of flue gas is reduced from 100m3 before absorber to 90m3 after absorber. The difference of 10m3 is the amount of CO2 collected.
NB. The dot in the top left corner of tables 1 and 2 illustrates the pollutants.
Developing the formula to calculate the correction
The calculations below are based on the assumption that emission measurements should be performed after the last flue gas cleaning step and the emission limit value specified in the IED should remain.
Based on the principles illustrated above, it is obvious that the calculations should be based on the amounts of N2 and O2 that pass unchanged through the collection system. It should be noted that an additional O2 measurement (dry flue gas) is required as the only additional measurement.
The simplest calculations are based on dry flue gas, i.e. O2 and flow measured after the CO2 absorber are corrected with measured H2O:
(1)
Where:
QO2 is the oxygen flow through the aborber.
KO2 is the oxygen concentration. The asterisk (*) indicates the concentration at the inlet to the absorber.
Qfg is the flue gas flow. The asterisk (*) indicates the flow rate at the inlet to the absorber.
Note that a bias in the oxygen measurements can be critical, as the difference between the two measurements is expected to be around 1 vol.%.
As a check , the amount of CO2 collected can be calculated and compared to the amount of CO2 leaving the facility for storage or utilization (taking into account time lags, etc.).
The limit value for primary components must be assessed based on the measurement of mass flow after CO2 capture and take into account changes in the capture system. Here exemplified by NOx
First, the mass flow after collection is transferred to the mass flow before collection, i.e:
(2)
Where:
KNOx is the NOx concentration. The asterisk (*) indicates the concentration at the inlet to the collection system.
Qfg is the flue gas flow. The asterisk (*) indicates the flow rate at the inlet to the collection system.
By combining formula (1) and (2), the concentration of the primary parameter before the CO2 absorber can be calculated according to:
The primary measurement can then be converted to a reference condition as specified in EN 14181 in combination with the above formula.
The above formulas are based on values for dry flue gases. The formulas are the same for wet flue gases. However, please note that the calculation must be done at standard conditions. This means that a valid determination of the water content is needed. The advantage of this approach is that O2 is usually already measured on most systems.
Uncertainty
A calculation example is performed based on a NOx concentration of 165 mg/m3 and measured oxygen concentrations of 7.3 vol.% (after the CO2 absorber) and 6.6 vol.% (before the CO2 absorber). The example shows that a NOx measurement that has a standard deviation of 10 mg/m3 is estimated to increase to 11 mg/m3 if both oxygen meters have an uncertainty of 0.2 vol.%.
The same estimated uncertainty for calculations based on wet flue gases is 12 mg/m3.
Conclusion
It is possible to make a relatively simple conversion of concentrations measured after a CO2 absorber to the flue gas conditions before the absorber. These can then be compared to the concentration-based limit values set before the introduction of the absorber. The method can also be performed without significantly increasing the uncertainty of the measurement.
Summarizing
Placing the AMS after the CC system and adding an additional O2 measurement before Carbon Capture seems to be the best and cheapest option, as the additional O2 measurement is probably already available, and if not, they are often a relatively small cost to introduce. The method also makes it possible to either calculate the amount of captured CO2 or to use the captured CO2 flow to cross-check the reliability of the O2 measurement.
It’s still too early to say which direction the installations and/or regulatory requirements will take. What is certain, however, is that facilities will have to work through this issue in the near future if they plan to implement carbon capture – and we believe most do.
Contact Kenneth Vindum, CEO for more information: kvin@www.olicem.com or visit us at CEM 2023 in Barcelona, stand 54.
