###### 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.

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 dryO_{2} measurement must be taken and measurements taken after CC must be corrected for the capturedCO_{2} 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 100m^{3} of flue gas are illustrated in table 1 below. The example also illustrates the components of the flue gas before a carbon capture system.

The oxygen concentration in dry flue gas is calculated as:

When the 100m^{3} passes through the CO_{2} absorber, approximately 90% of the CO_{2} will have been removed from the flue gas. This is illustrated in Table 2 below, where 10m^{3} of CO_{2} has been removed.

The oxygen concentration in dry flue gas is calculated as:

Note that the volume of flue gas is reduced from 100m^{3} before absorber to 90m^{3} after absorber. The difference of 10m^{3} is the amount of CO_{2} 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 N_{2} and O_{2} that pass unchanged through the collection system. It should be noted that an additional O_{2} measurement (dry flue gas) is required as the only additional measurement.

The simplest calculations are based on **dry flue gas**, i.e. O_{2} and flow measured after the CO_{2} absorber are corrected with measured H_{2}O:

(1)

**Where:**

Q_{O2} is the oxygen flow through the aborber.

K_{O2} is the oxygen concentration. The asterisk (*) indicates the concentration at the inlet to the absorber.

Q_{fg} 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 CO_{2} collected can be calculated and compared to the amount of CO_{2} 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 CO_{2} capture and take into account changes in the capture system. Here exemplified by NO_{x}

First, the mass flow after collection is transferred to the mass flow before collection, i.e:

(2)

**Where:**K

_{NOx}is the NO

_{x}concentration. The asterisk (*) indicates the concentration at the inlet to the collection system.

Q

_{fg}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 CO_{2} 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 O_{2} is usually already measured on most systems.

#### Uncertainty

A calculation example is performed based on a NO_{x} concentration of 165 mg/m^{3} and measured oxygen concentrations of 7.3 vol.% (after the CO_{2} absorber) and 6.6 vol.% (before the CO_{2} absorber). The example shows that a NO_{x} measurement that has a standard deviation of 10 mg/m^{3} is estimated to increase to 11 mg/m^{3} 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/m^{3}.

### Conclusion

It is possible to make a relatively simple conversion of concentrations measured after a CO_{2} 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 O_{2} measurement before Carbon Capture seems to be the best and cheapest option, as the additional O_{2} 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 CO_{2} or to use the captured CO_{2} flow to cross-check the reliability of the O_{2} 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**.

*At Olicem, we bring CO*_{2} calculations into the DAHS system in a single solution. Correction to new emission limits, CO_{2} quality, uncertainty calculations and CO_{2} fragmentation are examples of some of the areas we have been working on.

_{2}calculations into the DAHS system in a single solution. Correction to new emission limits, CO

_{2}quality, uncertainty calculations and CO

_{2}fragmentation are examples of some of the areas we have been working on.

*For more information, please contact Sales Director, Troels Skov Moestrup:*

*E-mail: tsm@www.olicem.com*

*Mobile: +45 21 49 57 18*

*For more information, please contact Sales Director, Troels Skov Moestrup:*