Advantages
and Issues –CO2 from
normal combustion (using air), is present as a dilute fluid in the flue gas
stream, which turns out to be an expensive method to capture carbon dioxide
from the nitrogen rich mixture through amine absorption. CO2 capture can be
accomplished by combusting fuel with oxygen to get a concentrated co2 stream
that can be easily sequestered.

The
process is a thermally efficient process because there is little or no nitrogen
to heat. Besides NOx emissions are drastically decreased.

Despite,
oxygen extraction or purchase is expensive, oxyfuel combustion can be proven to
be cost efficient since there is no need for additional furnace due to the high
energy efficiency of the process. 

Another
advantage is its potential to be retrofitted to existing plants, also it could
be allowed in a new plant for retrofit at a later stage, very environmentally
friendly due to the low NOx emissions and sequestration of CO2 for transportation
and storage.

On
the other hand, the disadvantages of the technology are also mentioned in this
paragraph. For instance, there is a lack of information as there are no
full-scale plant using oxyfuel combustion but pilot scale experiments. Another
point not in favour, is the cost for additional units for the desulphurisation
process. It is important to mention that air separation is an energy intensive stage
within the process for this reason the overall efficiency is less compared to
the one currently used pulverised fuel technology.

Best
Fit – It can be
considered as a cost-efficient solution when clients are willing to increase
performance or reduce Nitrogen oxides emissions. It may be the best fit for
plants where higher production is desired (30%) as well as reducing the cost of
fuels for up to 15%. Resulting in doubled profit margin and shorter payback
times, however if reducing fuel costs is the only target, this technology might
not be appropriate.

Critical
Review –  Experiments carried out using this technology
have encountered some design issues, they can be categorised according to their
origin.

The
first, is about design problems related to heat transfer. These problems are
caused due to gas thermal capacity (pure oxygen is combusted) and gas radiative
qualities, the latter is caused by the increased concentration of COx and water
vapour in the furnace which will cause the concentration of triatomic molecules
to drastically increase in the flue gas thus modifying the emissivity of the
gas.

The
second issue is related to the emissions, clearly the amount of NOx emissions
has drastically decreased, however the amount of sulphur remains unchanged. In
the final stage, compression and liquefaction of CO2 will generate a
non-condensable stream of components that could leak from air, since they get
recirculated these could build up, for this reason this issue needs to be
addressed in order to meet the equipment regulations along with environment
requirements.

The
third issue is related to ash accumulation that if recirculated could increase
the combustion temperature and hence modifying the boiler operability, a last
issue is related to ignition and flame strength, this type of stability-related
issue is supported by experimental data from the pilot plants.

To
address these problems accurately, more funding should be allowed for R&D
now because corporations do not measure the effect of their conventional plants
on the environment, it may be late when action is taken. The solution from a
general point of view is to increase research so the process is finally
implemented in a full-scale plant and not just stay in literature as a ‘near
zero technology’ that was only experimental.