4 Oxy-fuel combustion technology

Oxygen fired process is based on the combustion of pulverized coal (or other primary fossil fuels) using as oxidizing medium a mixture of oxygen and recycled CO2 rich flue gas, instead of air.

As no nitrogen is fed to the furnace, the flue gases consist mainly of CO2 (70-80%wt), water (10-15%wt) and inerts. After cooling, for removing the moisture condensate, approximately 65% to 70% of flue gas is recycled and mixed with oxygen to form a primary and secondary flue gas recycle stream that support coal combustion in the boiler.

The balance of the total exhaust gas from the boiler is fed to a CO2 purification and compression unit, where the water and inerts are removed. Then, purified CO2 can be sent to storage.

Design features, and consequently flexibility, of the oxy-combustion plants are in line with those of conventional air-fired boiler plants, as described in the previous section.

The capability of this technology to operate flexible is mainly affected by constraints on the Air Separation Unit and the CO2 purification and compression plant, as far as minimum turndown, start-up time and ramp rates are concerned.

As for the IGCC and the conventional PC boiler power plants with amine-based CO2 capture, the possibility of varying the power production in response to the changes in the electrical grid demand, tuning the internal power consumption, is investigated in the next sections.

4.1 Flexibility feature

4.1.1 Start-up sequence

One of the main features of the oxy-fuel power plants is that start-up and shutdown is made in air firing combustion mode. This allows to make the start-up of the boiler in air mode, while cooling down the Air Separation Unit.

The maximum load level that can be achieved with air firing is dependent on the load which the burners accept. In fact, in order to minimise uncontrolled emissions from the plant during switch over to oxy-fuel, it is advisable to operate at the lowest possible load, which generally is about 30%.

In the air-firing phase, the boiler load is increased to the minimum stable load using a back-up fuel (typically fuel-oil). At the same time, the steam turbine is heated, accelerated, synchronized and ramped up to minimum load. Boiler exhaust gases are sent to the stack, without being treated in the CO2 processing unit.

When both the boiler and the steam turbine are in operation at minimum stable load and oxygen from the air separation unit is generated at the required purity, the combustion mode is changed from air to oxygen and simultaneously the flue gas recirculation is started.

While increasing the plant load, also the switch over from back-up fuel to coal (or other primary fuels) is carried out. At the same time, CO2 compression unit is started-up. When plant load is increased to an acceptable value for the compressors, flue gas is fed to the CO2 purification and compression section.

4.1.2 Start-up time

Typical start-up time for the Air Separation Unit necessary to reach the required oxygen purity, in this case 95%, are summarised in the following Table 4-1.

Table 4-1. Start-up times for ASU in oxy-fuel plant

Initial condition Start-up time
After defrost 36 hours
After 24 hours shutdown 6 – 8 hours
After 16 hours shutdown 4 – 6 hours
After 8 hours shutdown 3 – 5 hours
Less that 1 hour shutdown Less than 1 hour

It has to be noted that, as the burners in the furnace are able to operate also under air-firing, hence with an oxygen purity of approximately 23%wt., it is possible during the transient to supply oxidizing agent to the boiler system, even with oxygen content lower than the design specification of 95%. This can be achieved by properly adjusting the recycle ratio in order to provide the correct temperature control and oxygen excess into the boiler. The only detrimental effect will be a reduction of plant capture performance and therefore on the amount of CO2 that can be captured, due to the inert content increase into the gases fed to the CO2 purification system.

It is noted out that the total time required to start-up the oxy-fuel power plant and change from air firing to oxy firing is not yet shown in literature data.

4.1.3 Ramp rates

Main limitation in cycling operation of the oxy-fuel combustion plant is given by the Air Separation Unit ramp rate.

The maximum ramp rate for an ASU is typically 3% per min, while for the boiler it is generally 6% per min. Therefore, a plant ramp rate in line with the boiler capacity can be achieved by using a dedicated and properly designed oxygen storage.

4.1.4 Turndown

Air Separation Unit turndown depends mainly on Main Air Compressor (MAC). These compressors operate efficiently in the range 70-100% of maximum flow. The cryogenic air distillation equipment is able to turn down at lower load, maintaining a constant oxygen recovery. This characteristic gives flexibility to operate efficiently in the range 70% to 100% with a single train configuration. Considering multiple train configuration, efficient operation is possible even at lower load.

If it is required to run below 70%, this has an impact on the machine's efficiency, as the following operational modes could be required:

  • Recycling a portion of the compressed air back to the inlet of the main air compressor;
  • Venting a portion of the produced oxygen;
  • Producing a certain quantity of liquid oxygen for backup storage, if foreseen.

CO2 compressor systems are capable of efficiently turning down to about 70% of full flow at constant discharge pressure. Operation at lower load can be achieved using a multiple train configuration or recycling part of the CO2.

4.2 Tuning power consumptions

As for conventional PC boiler plant with post-combustion capture, reducing the internal power consumption, when electricity prices are high, allows to follow the seasonal or daily market trend and participate in ancillary services markets, therefore increasing the remunerability of the plant.

In oxy-fuel power plants, the power consumption related to the CO2 purification (including compression) and to the cryogenic separation of oxygen in ASU is significant.

Therefore, to reduce energy penalty, a possibility is to change the boiler operation from oxy fired to a traditional air fired, when electricity demand rises. This approach can be followed depending on the variability of CO2 emissions cost: until the cost of emitted CO2 remains low, as in the present market conditions, it could be economically convenient to release the CO2, rather than limit the plant flexibility.

Currently, the oxy-fuel power plants are designed to allow flexible operation both in air and oxy-modes.

The main parameter influencing the boiler capability for a flexible and efficient operation in both firing modes is the flue gas recirculation flowrate, as some boiler design features, like furnace surfaces and boiler cross-sectional area, and operating parameters, like the combustion temperature, depend on the amount of flue gases in furnace.

Operation with high flue gas recirculation and an oxygen concentration around 30% leads to a flue gas amount in the combustion chamber that replaces the combustion air in the conventional boiler.

The flue gas treatment system downstream the boiler has to be sized for the proper flue gas flowrate, to achieve full capacity operation in both firing modes.

Also, providing liquid oxygen storage would temporarily avoid the operation of the ASU, increasing the electricity exported to the grid with the plant still operating at full load. In fact, by over-sizing the ASU, it is possible to produce extra O2 during periods of low electricity requirements from the market, providing storage of liquid product, while increasing the auxiliary consumptions. When the market requires a higher electricity generation, the ASU can be operated at partial load, while the rest of the plant is running at full load. This reduces the auxiliary consumptions, increasing the net electricity exported to the grid.

The increase of investment cost is related to the extra-capacity required for the ASU and oxygen storage facilities.

On the other hand, the alternative of storing the CO2 rich stream (upstream CO2 purification) for avoiding the energy penalty associated to the CO2 compression, without increasing CO2 emissions from the plant, is more difficult and it has not been evaluated yet.