5.1 Increased flue gas recirculation

Flue gas recirculation is already used at several waste incineration plants to regulate the pre-burning process and control NOx emissions. In connection with retrofitting a CO2 capture plant, it may be appropriate to take a closer look at the extent to which such recycling can be maximized and increased beyond today's levels. Increased recycling will have the following positive effects on a downstream CO2 capture plant:

• Increased CO2 concentration. This makes CO2 capture more efficient since it is easier to capture from high concentrations than low concentrations. You get more CO2 per kg of flue gas, and you need less flue gas. You can capture more CO2 per liter of capture chemical.
• Reduced flue gas quantity. This results in less flue gas volume going through the absorber tower and thus less diameter of absorber. A smaller absorber diameter results in lower investment costs, smaller chemical volumes, and possibly lower energy consumption for flue gas fans. Smaller flue gas quantities also have a positive effect on the cooling requirement since there is less hot flue gas that must be cooled before the absorption step of the capture process.
• Reduced O2 concentration. Oxygen is an undesirable component in an amine-based CO2 capture plant since oxygen contributes to degrading many of the most common capture chemicals. Increased recycling will therefore in many cases contribute to lower chemical consumption and lower operating costs for a capture plant.

One disadvantage may be that it can lead to increased CO concentration in the flue gas.

According to Hitachi Zosen Inova AG, it should be possible to reduce oxygen levels in a modern waste incineration plant down to 3-5% O2 with the help of flue gas recirculation.

The potential for "KAN Referansa" has not been calculated, but for players with a relatively high O2 level in the chimney, it may be appropriate to initiate a dialog with the plant supplier to discuss the potential of an increased proportion of flue gas recirculation. Moderate investments in flue gas circulation can potentially have a positive impact on investment and operating costs for a CO2 capture plant.

When establishing a new incineration line or waste incineration plant, the possibilities for flue gas recycling should be investigated more closely in consultation with suppliers. For existing lines, it may be worthwhile to have a dialogue with suppliers of furnaces and the combustion chamber. In existing or older facilities, this will be complicated and will lead to changes to the control system that may be difficult to regulate.

5.2 Oxygen combustion plants 

Several capture suppliers deliver what they call 'oxy-fuel combustion capture', where the combustion air has been replaced with pure oxygen. This increases the CO2 concentration in the flue gas, as well as reducing polluting gases with nitrogen (such as NOx), in theory the flue gas will only consist of H2O and CO2. The challenge is the source of oxygen, it most often comes from a cryogenic air release process (power-intensive). A less energy-intensive alternative to this that is under development is the Chemical Looping Combustion (CLC), where a metal oxide is circulated as an oxygen carrier for the combustion process.

Oxygen incineration plants may be worth investigating if a new fuel line is created, the benefits being that you reduce the number of pollutants, which apparently is not the thesis for waste incineration.